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

Use --depth option to avoid pulling devel branches

- fzf-tmux
- CTRL-T of bash/zsh/fish
    - fish implementation may not work if the path contains
      double-quote characters (FIXME)

Refactor shell extensions

- Use symlinks instead of generating the full content
- Update fish_user_paths and remove ~/.config/fish/functions/fzf.fish
- Create wrapper script for fzf when Ruby version and use it instead of
  exported function not to break fzf-tmux

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

[fzf-tmux] Allow opening fzf on any position (up/down/left/right)

The previous -w and -h will be synonyms for -r and -d respectively.