Display Text Directly wót Lukas Mai

Some files are served with a text/x-foo Content-Type, e.g. http://tomaw.net/irssi/printlevels.pl has Content-Type: text/x-perl. Instead of treating these unknown text types as text and displaying them, Firefox treats them as completely unknown and offers them for download.

This add-on rewrites text/x-* into text/plain so everything works as expected.