Allow adding arbitrary document code

[soraxas]

I'm using this PR as follows:

pic.usepackage("tikz-3dplot")

pic.add_document_code(r"\tdplotsetmaincoords{70}{110}")
pic.add_document_code(r"\tdplotsetmaincoords{60}{90}")

where

\tdplotsetmaincoords{70}{110}

is not part of the preamble (which is supported currently), but it needs to be in-between

\begin{document}
...
\end{document}