Better error handling for the conf header

Adds several rules in the lexxer that emit much better errors for the
conf header. When rules are broken for either part of the conf header,
instead of blowing up and jumping down to a general error, each section
now has its own error state. More descriptive output could still be
added but this is a good start.
Goodbye Doxygen.

For now, no documentation generator. Eventually, most of this code will
be pulled into a library to abstract the CLI functionality from the
actual parser and lexxer. When that happens, docs will likely be
generated by Docorium, not Doxygen. I mean, if Docorium is good enough
for libgit2, it sure as hell is good enough for libmt. ^_^