Allow the special colors 32..41 in %Cfg,bg

Text events allow %Cfg,bg as decribed in "Theming" on
https://hexchat.readthedocs.org/en/latest/appearance.html?#theming

That section describes mIRC colors 0-15 and local colors 16-31, but
when selecting background colors, the 9 special internal colors are
extremely useful, like %C20,33 will do bright red on the selected
text background color.

As the documentation is on-line, this patch has no doc-change.
Ideally, the color-codes should show in the colored squares in the
preferences color-dialog just as with colors 0-31

Signed-off-by: H.Merijn Brand <h.m.brand@xs4all.nl>
Merge pull request #594 from Tux/master

Allow the special colors 32..41 in %Cfg,bg