Add option to set parcoords context line color and opacity

[archmoj]

The deselected lines in all parcoords plots now use a constant color (i.e. #777 and auto opacity logic (i.e. depending on the number of overlaid lines).
In some scenarios it may be useful to let the user override the default.

[chriddyp]

If we could set the opacity to zero, then the user could have the unselected lines completely removed when filtering. This would be very nice for cluttered plots. for inspiration, see how hiplot does this in their parcoords tool (https://facebookresearch.github.io/hiplot/_static/demo/ml1.csv.html?hip.color_by=%22valid+ppl%22&hip.PARALLEL_PLOT.order=%5B%22dropout%22%2C%22embedding_size%22%2C%22epoch%22%2C%22lr%22%2C%22train+f1%22%2C%22valid+ppl%22%5D)