Highlighting

[mrjacobbloom]

see #255

This is my implementation of block highlighting. I've added the changes people requested. Namely:

• [x] prettier highlighting for the blocks with rounded edges
• [x] highlight both at the same time
• [x] turn it on or off in the new View menu

all the commits from #258 got attached to this PR which is lame, but if you compare the branches the only changes are ones relevant to this PR

[mrjacobbloom]

closing in favor of #263