Label rectangle for Edge

[liamross]

Is your feature request related to a problem? Please describe.

1. No way to display label text for Edges

Describe the solution you'd like

1. Find mid-coordinate of path (math things)
2. Put rectangle there with white background and label text
3. Maybe as a nice to have border should match edge color

Describe alternatives you've considered

NO ALTERNATIVES JUST DO IT

Additional context

This is low priority, as it's simply a nice-to-have implementation. Technically, anyone can implement this in their own custom shapes library built on-top of Psiagram

[liamross]

This question breaks down some of the logistics:

https://stackoverflow.com/questions/15500894/background-color-of-text-in-svg

[liamross]

Update: completed text positioning for TextLine, still needs rectangle