Allow people to specify/choose the theorem they're trying to prove

[NicMcPhee]

After completing #72 where people can select rules from a dropdown, and possibly as part of #69 (supporting Markdown output) it would be nice if people could specify what they're trying to prove at the top, with support for selecting from a dropdown menu of known rules.

If we did this right we could also support confirming that no later rule was used in a proof (e.g., you can't use rule 37 in proving theorem 24), but that might want to be a separate issue if/when we get there.