* 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
