Automatically populate the location list with errors from Syntastic.

This helps ensure that the correct contents are there when using the
shortcut to pop open the location list.
Add better syntax checking for Sphinx-based reStructuredText documents.
Enable active checking for reStructuredText documents.

Now that we have better checking for Sphinx-based documents, there's no
reason to avoid checking reStructuredText documents.
Allow warnings for the rst2pseudoxml checker.
Adjust :L, :L2, etc., for 82 columns (leaving room for signs).
Update to bufexplorer 7.4.5 (nnoremap patch has been merged).