highlight: only pygmentize for HTML mimetypes
For non-html mimetypes it doesn't make much sense. This also fixes the
issue that highlight unconditionally adds a <link/> tag for its CSS to
the template's header (which is pointless in text/plain output).
% basic
% unknown
*** [alias] unknown: command bargle is unknown
% ambiguous
*** [alias] ambiguous: command s is ambiguous
% recursive
*** [alias] recursive: circular dependency on recursive
adding foo
% with opts
C foo