Handle uppercase letters in program names. This also deals with `-` and
`.`, both of which are quite common in program names, e.g., `xdg-open`
and `foo.sh`.

The command substitution and following word splitting to determine the default
zle widget for ^I formerly only works if the IFS parameter contains a space. Now
it specifically splits at spaces, regardless of IFS.

Close #607

Close #615

Related: #616

Close #611

zle automatically calls zle-line-init when it starts to read a new line. Many
Zsh setups use this hook to set the terminal into application mode, since this
will then allow defining keybinds based on the $terminfo variable (the escape
codes in said variable are only valid in application mode).

However, fzf resets the terminal into raw mode, rendering $terminfo values
invalid once the widget has finished. Accordingly, keyboard bindings defined
via $terminfo won’t work anymore.

This fixes the issue by calling zle-line-init when widgets finish. Care is taken
to not call this widget when it is undefined.

Fixes #279

This allows to have a custom widget like the following, which would
additionally accept the line, but only in case of entries being
selected:

    fzf-file-widget-with-accept() {
      zle fzf-file-widget
      if [[ "$?" == 0 ]] && (( $#BUFFER )); then
        zle accept-line
      fi
    }
    zle     -N   fzf-file-widget-with-accept
    bindkey '\e^T' fzf-file-widget-with-accept

With this `<C-a>t` will launch fzf, and simulate the pressing of "Enter"
afterwards.

Close #596

Signed-off-by: Junegunn Choi <junegunn.c@gmail.com>

Close #580

e.g. Remapping fzf-file-widget to CTRL-X CTRL-T intead of CTRL-T

    bind -x '"\C-x\C-t": fzf-file-widget'
    bind '"\C-t": transpose-chars'

- fzf-file-widget
- fzf-history-widget
- fzf-cd-widget

Related: 23244bb4

Faster startup. Use internal bash globbing instead of external grep binary (adapter from Gentoo's `/etc/bash/bashrc` TERM checking). Insignificant on Linux, but on Cygwin this cuts startup time by 40 ms on my Core i7 laptop.

Fixes #548. Avoid using a subshell in _fzf_defc().

Summary:
Fix adapted from [@adamheins: fzf, vi-mode, and fixing delays][1].

  [1]: https://adamheins.com/blog/fzf-vi-mode-and-fixing-delays

The basic problem is that
fzf presses <Esc> to enter vi-movement-mode
(as opposed to insert mode)
and then presses a bunch of keys to set up the buffer.
But the <Esc> keypress is also the prefix for a bunch of other commands,
so Bash will dutifully wait an excruciating half-second
before actually executing this command.
Instead, we bind <C-x><C-a>, which is unused by default
and seems reasonably unlikely to be custom-bound,
to be another way to enter vi-movement-mode;
this binding is unambiguous, so fzf can use it without delay.

This change was made by just `:s/\\e/\\C-x\\C-a/gc`
in the relevant section,
after adding the actual binding and comment at the top.

Close #526

Close #516

e.g.

_fzf_complete_foo() {
  _fzf_complete "--multi --reverse --header-lines=3" "$@" < <(
    ls -al
  )
}

_fzf_complete_foo_post() {
  awk '{print $NF}'
}

[ -n "$BASH" ] && complete -F _fzf_complete_foo -o default -o bashdefault foo

Notes:
- You can now override _fzf_compgen_path and _fzf_compgen_dir functions
  to use custom commands such as ag instead of find for listing
  completion candidates.
    - The first argument is the base path to start traversal
- Removed file-only completion in bash, i.e. _fzf_file_completion.
  Maintaining a list of commands that only expect files, not
  directories, is cumbersome (there are too many) and error-prone.

TBD:
- Added $FZF_COMPLETION_DIR_COMMANDS to customize the list of commands
  which use directory-only completion. The default is "cd pushd rmdir".
  Not sure if it's the best approach to address the requirement, I'll
  leave it as an undocumented feature.

Related: #406 (@thomcom), #456 (@frizinak)

This seems like a bug of fish, but sometimes when you select an item
fish complains:

"insertion mode switches can not be used when not in insertion mode"

This only happens when using tmux pane. Injecting a dummy command
somehow fixes the issue.

Close #456 and #457

Instead of choosing one at initialization, choose the correct one
when it's actually called, so that the behavior is correct even after
resizing.

Bonus fixes for tmux with bash 4+:
- No extra space when cancelling CTRL-T.
- Fix cursor position problem in vi mode.

- Don't assume ~/.fzf.bash exists.
- Source the current script for __fzf_select__.
- Forward $PATH.