Added speedup improvement

Adds a speedup improvement, that can be quite high with markdown strings without too much markup.

It works like this:
- We create a string buffer for the current token.
- Every time that we consume the same token as before, we just add it to this string buffer, and we dump it to the attributed buffer (``_accum``) every time that we get a different one.

Doing this avoids allocating a new ``NSAttributedString`` and their associated attributes every time that we read a token (that can be just one letter for unformatted text), and the overhead of adding it to the existing parsed text.