Add support for upper and mixed-case bbcode tags

The current parser only handles fully lowercase bbcode tags. This change
augments the existing functionality to essentially make the bbcode tag
detection case insensitive, for cases where you may have upper or
mixed-cased tags in your source content.

I'm not enough of a bbcode expert to know whether upper/mixed case tags are 'legal', but I have some source content with that formatting, and so needed these changes in order to make the module fully compatible for my purposes. Hopefully you find the change useful and want to pull it back into your master copy.

I also added a few quick tests to verify all is working well.

Let me know if you have any questions.

Dan