Optimized text generation.

The way the code was implemented before, each block of text was
generating a string which was then being added to a StringBuilder. This
only improved performance within a block itself. Needing to then copy
the results of that builder into the parent tag's builder was wasteful.
Now, a single TextWriter is used for all tags. If a block needs to be
processed after-the-fact, the tag can indicate that it wants to provide
a new text writer and that it wants to consolidate the text.