Improve search support

[AustinWise]

It should be easy to search for content in the wiki. Even basic searching is useful, but there is a lot that can be done to make the experience great:

• Index files on startup and allow searching #23
• Update search index on document change #41
• In search UI, update search results on every key press
• Support regex queries
• Consider persisting search index to disk. Currently it is in-memory and recreated on every startup.
• Speed up indexing. Currently it is a little slow when indexing tens of thousands of documents.