visitedTag() + minor fixes

* Added visitedTag(), which returns the number of times passages with
the given tag have been visited. Provide multiple tags to only search
for passages that contain both.
* Formatting syntax inside link text is once again no longer parsed.
* Added word-break: break-all to #sidebar li, to keep unnaturally long
words from overflowing the sidebar.
* Built HTML files are now timestamped.