Some new count_biotypes.py features.

Group low frequency biotypes into 'other', made special cases a grey colour and write in the ability to "translate" biotype labels easily. Added flag to include reads with no feature overlap into plots.

Fixes SciLifeLab/visualizations#6.
Ah sure. I've probably done that more than a couple of times :grin: 
Using ```lstrip().rstrip()``` instead is no big deal... I was just curious.