cmus-info shows filename when no tags available

I need to chance query-cmus to accommodate for when there are no tags
available before this works as desired. Currently it just doesn't show a
filename. This isn't too different from what it currently does, but it's
still not done.
cmus-info now works with no tag info

I updated cmus-query to be more generic, then made a helper to make
querying tag info easy, which I called tag-query. I also wrote a helper
called emptystringsp which returns whether the list of strings contains
any empty strings.

This isn't completely ideal, since if either title, artist or album is
empty it will only show the filename. I'm not really sure precisely
which representation would be best, but this is at least better than
getting no information about a song that's missing tags.
Fixed tag-query so it only grabs the right tag

Before, given the input 'album', it'd match the tag 'album' and
'albumartist', now it only matches 'album, as expected.
Updated calls to query-cmus

Just realized other aspects will have been calling query-cmus with tags,
so I updated them to use tag-query instead.
Merge pull request #3 from futuro/master

cmus-info now works with no tags - Thank you for your contribution - the old cmus-query was admittedly kludgy, at best.