Add control + left/right key mappings

On Ubuntu/Debian, completion functions can be dynamically loaded via
_completion_loader. Since those functions are not visible when
fzf-completion.bash is loaded, we need this special hack to make it
possible to fail back to the original completion function when trigger
sequence is not found.

Old versions of sed does not have -E option

Although a major overhaul is ongoing (#67), it is not yet finished and
cannot be considered stable enough for the next release. This commit
fixes a few apparent issues with small change to the current
implementation.

- Fixed error when $TMPDIR is not defined
- Better escaping of file/directory names
- Splitted functions to workaround fish bug

[bash-completion] Fail back to original completion

Add options: --prompt and --print-query

Make the list scrollable

- Suppress warning message on Ruby 1.8.5
- Remove unnecessary code

Add vi-command keymap mappings

fzf does not currently define vi-command mode mappings. This is particularly annoying for <C-r>, which opens bash's old-fashioned recursive history search.

This patch adds vi-command mode mappings that simply drop back into vi-insert mode ("i") and then trigger the primary mapping.

Use `command find` rather than plain `find`

Aliases are expanded in shell scripts, and one may have an alias
for the `find` command that conflicts with fzf. So make sure fzf
is using real find command rather than the alias.

Search is interrupted when the query string has changed. This frequently
happens when the user is actively typing in a query. This (rather
arbitrary) delay is introduced not to start the next search immediately,
which is likely to be interrupted as well. The result of it is that fzf
feels more responsive.

As discussed in #55