* Add MSYS2 support as a vim plugin

Add &shellcmdflag and TERM environment variable treatment.

- Make &shellcmdflag `/C` when &shell turns into `cmd.exe`
- Delete %TERM% environment variable before fzf execution

* Change shellescape default value depending on s:is_win flag

* Make TERM environment empty only when gui is running

* Stop checking &shell in fzf#shellescape function

This funcion's behavior is controlled by only if it is Windows or not.
So there is no need to check &shell.

* Take neovim into consideration when to set shellcmdflag

* Add &shellxquote control

Close #1776

Backport https://github.com/junegunn/vim-plug/pull/913

Close #1787
Related #1364

If the command template doesn't have any placeholder expressions.

    : | fzf --bind 'space:reload:seq 10'

This commit fixes a bug where lines that declare multiple hostnames get
omitted from completion entirely if one of the hostnames matches *. For
example:

	Host foo.com bar.dev baz.*

Close #1783

Close #1707
Close #1779

* Update completion.bash
* Update completion.zsh

Added NixOS instruction. Close #1731

The zsh version of the cd widget sets the variable `dir` to the path of
the target directory before invoking `cd`. This causes zsh to treat the
target directory as a named directory, which has the effect of zsh
substituting '%~' with '~dir' instead of the proper path when it
performs prompt expansion.

This commit will cause the widget to unset `dir` before redrawing the
prompt to fix this issue.

Details of zsh prompt expansion can be found in:
http://zsh.sourceforge.net/Doc/Release/Prompt-Expansion.html

  # Note + prefix in the second bind expression
  fzf --bind u:up --bind u:+up
  fzf --bind u:up+up

Close #1699

Close #1738

Close #1752

In recent Nvim versions, an "Error running ..." message is shown even for normal
use cases, such as:

    :Files
    <c-\><c-n>
    :close

Closing the window will :bwipeout! the terminal buffer, because fzf sets
bufhiden=wipe.

When deleting the terminal buffer while fzf is still running, Nvim sends SIGHUP.
This happens for quite some time already, but the bug only manifests since this
commit:

  https://github.com/neovim/neovim/commit/939d9053b

It's The Right Thing to do when the application exited due to a signal.

Before that commit, no "Error running ..." message was shown, because 1 (instead
of 128 + 1 == SIGHUP) was returned which the exit handler in fzf.vim treats as
"NO MATCH".