Make labels cross-browser compatible.

Percent-encoded URL fragments prevent Firefox from jumping to their respective anchors (see, e.g., [RDoc's History.txt](http://docs.seattlerb.org/rdoc/History_rdoc.html): None of those links in the table of contents work in Firefox). Wikipedia (MediaWiki) does something similar, with `.` instead of `-`.

There's precedent in `RDoc::{Alias,MethodAttr}#html_name` (54dbcf70) and `RDoc::Context::Section#aref` (431baa1b). This breaks backwards-compatibility, though.