more complete medium space (\:) support

this change makes it so you can:
 1. type \\: and get a medium space (as a bare " " gives you now)
 2. give mathquill latex with \\: in it and have it render it correctly. previously it would display as "\\:" instead of " ", since it wasn't a recognized command. this was annoying, as mathquill was unable to render latex it had previously generated

it seems like other whitespace commands could be implemented in a similar fashion. if there's a better approach, let me know.