1. Go binary: ../bin/fzf
2. System fzf: $(which fzf)
3. Download fzf from GitHub or create wrapper script to Ruby version (../fzf)
   when the binary for the platform is not available
4. If install script is not found or for some reason failed, try to use Ruby
   version in its expected location (../fzf)
5. If fzf is found to be a shell function, use it (type fzf)

Close #189

Because tmux panes are not used on Neovim.

Ruby version can also accept `--expect` option although it's ignored.

It is already handled by its caller.

It is possible that the user has an autocmd that changes the current
directory.

Suggested by @noahfrederick

Prior to this change, you'd get a longer error message if you did:

    :FZF
    <esc>
    :FZF

The main problem being that `:file [FZF]` can be used only once.

Use terminal emulator of Neovim to open fzf

When 1 is given, 50% of the screen width or height will be used as the
default size of the pane.

As they're made synchronous with the use of fzf-tmux script

This avoids the problem in which :FZF command silently fails when fzf
executable cannot be found in $PATH of the hosting tmux server.

This makes it easier to override FZF command like follows:

    autocmd VimEnter * command! -nargs=? -bang -complete=dir FZF call fzf#run({
          \ 'sink': 'tabe',
          \ 'dir': <q-args>,
          \ 'options': '-m',
          \ 'tmux_height': empty('<bang>') ? '40%' : '' })

Fixed escaping bug of the previous commit

This reverts commit cc9938d4.