`find` should accept a list of tracks

Currently (v1.15), find searches only one track and defaults to the first suitable one. Maybe better is to allow a list of track-capturing regexes (i.e., like most other commands), search each track and stop at the first match found.

E.g., the following searches for ACTB starting from the track(s) matching foo.gtf, if nothing is found move to bar.bed and then spam.gff. baz.bam is silently skipped.

find ACTB foo.gtf bar.bed baz.bam spam.gff