allow selecting the backend by setting the environment variable MPLBACKEND

This commit adds a new way of selecting the matplotlib backend. The `-d` flag can cause problems with scripts that use the command line arguments as parameters and don't ignore unknown ones. An environment variable `MPL_BACKEND` can be set by the user (or in the code) and is far less likely to cause problems than using the `-d` flag.
@mdboom, just as a sanity check, are we going to have to worry at all about string/unicode issues here?