Recognize doctest options defined in other plugins.

I needed to use the [doctest-ignore-unicode](https://github.com/gnublade/doctest-ignore-unicode) plugin, but found that `--doctest-options='+IGNORE_UNICODE'` would fail.

The problem is that the doctest plugin fetched flags via `getattr(doctest, flagname)` instead of `doctest.OPTIONFLAGS_BY_NAME[flagname]`, which means it won't detect doctest options registered by other plugins.

This patch fixes that.