Preamble section patch

[Lorenzo Bertini]

> Can you give more details on what you mean by "far from perfect"?
Yes, sorry! The thing I'm not happy with is that patch makes it so the 
HTML preamble gets stored in a new bufferParams docstring, but it 
doesn't get checked for illegal characters and it's "brutally" printed 
in writeHTMLSource in buffer.cpp, while the LaTeX preamble string 
undergoes more processing in writeLaTeX in BufferParams.cpp and 
writeLaTeXSource in Buffer.cpp.

I wish I could dig deeper on LaTeX and LyXHTML outputting, but I feel 
like this is too hard of a start. Ill pick something easier next time.

Lo