Supports locale-specified encoding for rcfile.

This commit fixes issue #1317.  The locale for the encoding is obtained from
the Python `locale` module, which can be specified by the `LANG` environment
variable.

This was tested by adding a test rc file encoded in UTF-32-BE, and testing that
it decoded correctly.
Testing the module by invoking a "private" method is something I'd prefer not to do.

It looks to me like `rc_params_from_file` returns a new `RcParams` object every time, and I don't see them sharing any global state. If you are still concerned, would `del rc['timezone']` be enough to remove the configuration I've added?