Allow custom IFO prefix for Channels
Old LIGO FMCS channels have odd prefixes, but are still valid channels, e.g.
'LVE-EX:X3_810BTORR.mean,m-trend'
This PR introduces a unittest to make sure these channels can be parsed, and amends the Channel.MATCH
regular expression to allow custom prefixes.
This should fix #31 (closed).