Heatmap track filter

[eweitz]

This enables filtering tracks when annotations have a heatmap layout.

It extends "Filtering annotations by track" with support for heatmaps, which requires special handling as heatmaps render via canvas instead of SVG.

See #114 for a quick example of how this looks in action.

[coveralls]

Coverage increased (+0.003%) to 91.732% when pulling 62ac772 on heatmap-track-filter into 6cc82af on master.