Support custom graphs

[ZimbiX]

I recently forked this repo to add a graph for my home solar power. I think it would be nice to make this feature generic and upstream the change. I'm thinking of something as follows:

• Have a setting to specify a script to run each tick to generate the new data

• The script must output in an expected format, e.g. for my case, perhaps something like:

  production,6000,W
  total-consumption,1000,W
  net-consumption,-5000,W
  

  (I haven't yet worked out how to handle negative numbers in my graph, but that's beside the point)

• Have a way of selecting the colour of each plot. Perhaps the easiest would be via a fixed number of colour settings for line 1, line 2, etc.

[venteto]

The idea custom graphs support is interesting too