add strandedness to drawing

[bradfordcondon]

In short, we would like +/- strand subfeatures to be drawn with a pointed side indicating the strandedness of the feature, like JBrowse does.

https://github.com/statonlab/tripal_manage_analyses/tree/round_directoin_with_d3

I started this but forgot how annoying D3 is.

I have a branch with the g holding hte rect selected. i . think (?) the simplest way to get this is to use clippath to remove the top and bottom of the correct side, but i probably dont have the time i need to make this happen right now.

[bradfordcondon]

oops nevermind, just switch rect to polygon of class arrow. this is tricky though because tehy are drawn as rects so they just have width and hieght and we need to pass all 5 points as parameters