Generate multiple svg graphs

[glinesbdev]

This work is based on #530 and should be merged in after that PR.

Example Usage:

import graph_render

graph1 = graph_render.GraphData(['Time (ms)'], 'graph1.csv', 'First Graph')
graph2 = graph_render.GraphData(['Demographic', 'Currency'], 'graph2.csv', 'Second Graph')

graph_render.gen_graphs([graph1, graph2])

[avivace]

I'm on this, thanks @glinesbdev !

[avivace]

moved the comment in the other PR

[avivace]

Done in https://github.com/gbdev/pandocs/blob/master/src/imgs/src/preproc.py (PR #530)