Close #193

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.

install: wait for LF in non-interactive shell

"read -n 1 ..." ignores all but the first character of a line-delimited
stream (e.g. "yes n | ./install").

Check if fzf#run() is already executing

See: #181

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

Suggested by @noahfrederick

Check if :FZF is already executing

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

Close #175

So that it can be used with the recent Vim plugin although extra key
bindings are not available