Helps when your `$SHELL` is slow.

Close #552.

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

When we prepend a single quote to our query in --exact mode, we are not
supposed to limit the scope of the new search to the previous
exact-match result.

[bash] Update completion.bash

[bash] Update key-bindings.bash

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().

Fix https://github.com/junegunn/fzf.vim/issues/128

Based on the patch by Matt Westcott (@mjwestcott).
But with a more conservative approach:
- Does not use linearly increasing penalties; It is agreed upon that we
  should prefer matching characters at the beginnings of the words, but
  it's not always clear that the relevance is inversely proportional to
  the distance from the beginning.
- The approach here is more conservative in that the bonus is never
  large enough to override the matchlen, so it can be thought of as the
  first implicit tiebreak criterion.
- One may argue the change breaks the contract of --tiebreak, but the
  judgement depends on the definition of "tie".

[bash] Fix vi mode pre-launch delay

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.

https://github.com/junegunn/fzf.vim/issues/114

Fix algorithm tests

- Do not create zsh files if zsh is not installed (@adam8157)
- Use command -v instead of which (@netei)
- Reenable --pre option

Close #531

Fix typo in README.md

Close #526

Patch submitted by @netei. Close #524.

Related: https://github.com/junegunn/gv.vim/issues/19

Close #500