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

Updates MacVim wiki link in README

Prepending can be problematic when the user install fzf using Homebrew,
execute the install script, and later upgrade fzf with Homebrew, and do
not rerun the install script. In that case, even though the homebrew
package is upgraded, the older version will still be used.

$fzf_base/man may not exist when installed with Homebrew.

    tree -C | fzf --ansi --tac