Ignore common Sphinx-specific roles.

This is related to a recent [issue](https://github.com/scrooloose/syntastic/issues/1195) reported for syntastic.  The attached patch makes checking Sphinx documents somewhat more useful, by ignoring (most of the) standard Sphnix roles.  The patched `rstcheck` still doesn't do a great job parsing the [Hitchhiker's Guide to Python](https://github.com/kennethreitz/python-guide), but it does make that task less of a pain.