options: expand ~ for the include option

Fixes #1406
vf: fix crash if filter doesn't provide control entrypoint

This input command crashed:

    vf add @mf:format=yuv420p ; show_text "${vf-metadata/mf}"

Fixes #1408.
demux_playlist: unquote entries in pls playlists

I guess these parsers still have a way to go...
command: make empty vf-metadata not an error

If a filter exists, but has no metadata, just return success. This
allows the user to distinguish between no metadata available, and filter
not inserted.

See #1408.