Use lower-case letters in non-WYSIWYG editor.

References #122
Open/close tags, support for language-based tags in non-WYSIWYG.

Some plugins might want to hook into the non-WYSIWYG editor and
create buttons which do use different "tags" for open/close or
not create HTML markup at all. This makes it possible for them
to do so.

References #122
References #114