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).
This patch allows a per-repository (for example, within a hgwebdir) selection
of pygments_style web option. No static .css files required.
Test edited by pmezard (compatibility fixes)
