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.
<img src="http://www.quickmeme.com/img/27/272632b44bc663fd84aa0231d7aa73b78635e0b45b8f632fb708bb0fb20e178a.jpg" width=250>