Support channel filtering

[denis-stepanov]

In the cases where the channel being watched is fixed and known in advance, it would be good to have an option -f PREFIX which would filter hits by a prefix, enabling only those that match. Ee.g. -f FR_TF1_ would enable hits on TF1 jingles only. This would reduce cross-channel talk (false positives coming from another channel). Note that still all the channels would be probed by DejaVu, so it would not decrease system load. Multiple option invocations should be possible. A reverse option -F PREFIX should equally be possible, meaning "allow all but PREFIX".