Fix #1795

Truecolor does not work on default Windows terminal.
It is a problem in neovim GUIs.

https://github.com/sainnhe/edge/issues/5#issuecomment-565748240

* 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