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.
Good question. I'm sure I looked at this and thought the same myself, not sure why the change hasn't appeared. Must have `ctrl-z`ed it by accident..