lib/rdoc/markup/to_label.rb (convert): Make labels cross-browser compatible.

Percent-encoded URL fragments prevent Firefox from jumping to their respective anchors. 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.
Merge pull request #330 from blackwinter/make-labels-cross-browser-compatible

Make labels cross-browser compatible.