rst-option: writer

Currently the rst3 command with call_docutils=True keys on the extension of the "@rst filename.ext" node to determine the docutils writer to use. This does not allow using alternate or custom writers directly from leo.

Propose an rst option "writer" option, e.g. "@rst-option writer = latexmath" which defaults to None, falling back to the current inference scheme.

With this option it would also make sense to ignore the file extension if "writer" is set in methods like rstCommands.{write_files, writeToDocutils, etc}. This leaves it to the user to use an appropriate one.