Plotter color everywhere v2

[JaviMerino]

This is v2 of #199. Changes since v1:

• Added @derkling's suggestion to accept colours specified in hex format in the signals
• @derkling's suggestion of adding a group name for the colour in the signal regexp
• Improved signal's docstring as suggested by @derkling
• Moved to_dygraph_colors() to trappy/plotter/ColorMap.py as suggested by @sinkap

Since 35e4045 the static plotters (LinePlot mainly) support an additional colors parameter. Extend this support and interface to ILinePlot. Add support for it in the signal interface: trace_event:column:color with color a comma-spearated rgb value between 0 and 255. For example, thermal:temp:255,0,0 plots the temperature in the thermal trace in red.

[sinkap]

for the def to_dygraph_colors exactly what I had in mind! Thanks! :+1: