Fixes "E706: Variable type mismatch for: ret" when an exception is
caught.

Related: #470, #497

- GOPATH is no longer required
- fzf repository does not have to be in GOPATH
- Build Linux binary with Go 1.5.3

- Slightly more efficient processing of Options
- Do not return reference type arguments that are mutated inside the
  function
- Use util.Constrain function when appropriate

Go 1.3 compatibility

[man] Remove useless `.R` macros

If you do `man fzf > /dev/null`, you'll get the following output

`R' is a string (producing the registered sign), not a macro.
`R' is a string (producing the registered sign), not a macro.
`R' is a string (producing the registered sign), not a macro.
`R' is a string (producing the registered sign), not a macro.
`R' is a string (producing the registered sign), not a macro.
`R' is a string (producing the registered sign), not a macro.

Removing these `.R` macros with a newline seems to have no effect on the
page but gets rid of the error.

e.g. Opening another buffer when `set nohidden`

https://github.com/junegunn/fzf.vim/issues/77

Note that $SHELL only points to the default shell instead of the current
shell. If you're on a non-default shell, you might want to override the
value like follows.

  SHELL=zsh fzf --bind 'enter:execute:echo $ZSH_VERSION; sleep 1'

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)

fzf defers the initial rendering of the screen up to 100ms if the input
stream is ongoing to prevent unnecessary redraw during the initial
phase. However, 100ms delay is quite noticeable and might give the
impression that fzf is not snappy enough. This commit reduces the
maximum delay down to 20ms when --tac is not specified, in which case
the input list quickly fills the entire screen.

Related: #452

When `--multi` is set, tab key will bring your cursor down, and
shift-tab up. But since fzf by default draws the screen in bottom-up
fashion, one may feel that the opposite of the behavior is more
desirable and choose to customize the key bindings as follows.

    export FZF_DEFAULT_OPTS="--bind tab:toggle-up,shift-tab:toggle-down"

This configuration, however, becomes no longer straightforward when
`--reverse` is set and fzf switches to top-down layout. To address the
requirement, this commit adds `toggle-in` and `toggle-out` option which
switch direction depending on `--reverse`-ness.

    export FZF_DEFAULT_OPTS="--bind tab:toggle-out,shift-tab:toggle-in"

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.

This commit compensates for the performance overhead from the
extended tiebreak option.

When 'dir' option is passed to fzf#run(), the current working directory
is temporarily changed to the given directory, and restored at the end.
However, this behavior is not compatible with &autochdir. This commit
introduces a heuristic to determine whether or not to restore the
previous working directory.

Related: https://github.com/junegunn/fzf.vim/issues/70

We should not restore the previous working directory if the current
directory has changed somehow. This can happen when &autochdir is set.

Close #469. `setlocal nospell` should appear before `setf fzf` to allow
customization of the option.

Related: https://github.com/junegunn/fzf.vim/issues/67

Fix auto-completion for `/`

Close #456 and #457

Pass FZF_DEFAULT_OPTS to non-interactive bash instance