Consider: More than one line could occupy the same space

[shannonturner]

Worth considering a feature (requested on reddit) where two lines could occupy the same space. Related to, but distinct from #67 and #47, though likely if I can address those, this one becomes less important.

Pros:

• Could make overlapping lines look less awkward

Cons:

• Could significantly increase the amount of data required to store maps, with all of the side effects that has (larger SVGs, slower loading, easier to hit the max data cap)
• Adds significant complexity -- how do you erase without erasing ALL lines? (I'd have to do layers, and that makes it less user-friendly)