Related: #345

Fixes Helptags! command from fzf.vim

Values for 'up' and 'down' can be written with ~ prefix. Only applies
when the source is a Vim list.

    e.g. { 'source': range(10), 'down': '~40%' }

This reverts commit c601fc64.

Close #303

Close #303

To avoid conflict with other neoterm plugins that manage terminals,
prefer named terminals.

- bd! leaves the window open when there's no other listed buffer
- redraw! seems to help avoid Neovim issues.

/cc @alerque

- Ask for user confirmation before running `install --bin`
- Removed `s:fzf_rb` since `install --bin` will create a wrapper
  executable that just runs Ruby version on the platforms where prebuilt
  binaries are not available.

Code submitted by @lydell

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.