[completion] Revamp completion API

* _fzf_complete is the helper function for custom completion
    * _fzf_complete FZF_OPTS ARGS
    * Reads the output of the source command instead of the command string
    * In zsh, you can use pipe to feed the data into the function, but
      it's not possible in bash as by doing so COMPREPLY is set from the
      subshell and thus nullified
* Change the naming convention for consistency:
    * _fzf_complete_COMMAND

e.g.

  # pass completion suggested by @d4ndo (#362)
  _fzf_complete_pass() {
    _fzf_complete '+m' "$@" < <(
      local pwdir=${PASSWORD_STORE_DIR-~/.password-store/}
      local stringsize="${#pwdir}"
      find "$pwdir" -name "*.gpg" -print |
          cut -c "$((stringsize + 1))"-  |
          sed -e 's/\(.*\)\.gpg/\1/'
    )
  }

  # Only in bash
  complete -F _fzf_complete_pass -o default -o bashdefault pass