Provide a way to define handlers

You can now define handlers by creating a new HandlerMux. This can take
a HandlerFunc, which receives the BraaiTagNode as an argument and should
return a final string and an error. Additionally, there is a Handle
method which uses reflection to call methods on a struct as it
encounters them, erroring appropriately if an unknown method is
encountered in the Braai source.
Add Basic Support for Block Handlers

Block handlers can now be defined with HandleBlockFunc. They are
low-level, similar to HandleFunc. I'm planning on adding a
HandleBlockWrap, which provides facilities for handling the common case
where block handlers are used to wrap content with a prefix and a suffix
Remove Development Cruft

This was necessary before the ability to define handlers, as a way to
see the output of the parser. The removed String() and Html() methods
can now be implemented as Handlers.