Add customizable song format.

This patch lets the user to specify song rendering format with `:song-format <FORMAT>` command.

Supported features:

  * `%foo%` metadata queries. Expands into file tags if possible:

        %artist% => track artist
        %title%  => track title
        ...

  * `(foo|bar|baz)` (possibly nested) alternative layouts:

        (%title%|%directory%/%filename%) => track title or file path if title tag is not found

I don't particularly care about syntax though.

The default formatting does not change.

[Example][0]

  [0]: https://github.com/supki/.dotfiles/blob/03ecf0a77e4d9f53d669b694220f03238d8490cd/core/vimusrc#L20