Color the labels for new-style priorities, too

In more recent versions of Trac, the default priorities were changed
to blocker, critical, major, minor and trivial. This change includes
these new priorities in the label color scheme, matching them to
Trac's default color scheme as closely as possible.
Translate Trac-style hyperlinks to Markdown
Merge pull request #13 from ctrueden/hyperlinks

Translate Trac-style hyperlinks to Markdown
Merge pull request #14 from ctrueden/priority-colors

Color the labels for new-style priorities, too