Support for extending SpecialChar rule via plugin

I’m implementing superscript/subscript support via plugin. It works fine when there’s a space before the character starting superscript (like `some ^super^`), but to get it work even for e.g. `some^super^` (as in Pandoc Markdown), I need to add `^` to the rule SpecialChar.

I’ve extended plugins builder and parser so one can easily register some new special characters with plugin. If you like it, could you please merge it and release?

BTW: Superscript/subscript is actually pretty common extension so it could be useful even for other users. Are you interested in adding this into Pegdown as an extension?