Better handling of overlapping lines

[shannonturner]

From email:

One thing that might be nice is if tiles of the same line separated by one empty tile (including diagonally) would connect (drawing a line across the empty tile) so lines could overlap.

Related to #47, but distinct; I think increasing the amount of allowed space between connected lines might be a nice touch if I can do it well (even if it means re-generating images!)

[shannonturner]

Some helpful comments and suggestions on what users would like to see in https://www.reddit.com/r/TransitDiagrams/comments/1hf6h33/are_there_any_alternatives_to_tennessine_to_make/

[shannonturner]

Also requested on Youtube:

love the new features! For the next update, I would love to see the ability to be able to cross lines without them breaking. It's a problem on some Metro maps I'm creating, as on them when I cross them they get cut and make the line not look connected.

[shannonturner]

Also requested on Youtube:

The ability for a line to perpendicularly cross another line and remain continuous