Support themes inherited from sphinx_rtd_theme

[Lingnik]

versions.html checks to make sure html_theme == sphinx_rtd_theme. We created a theme based off of sphinx_rtd_theme, and had to copy a modified versions.html into our theme && remove the conditional in there in order to get the versions dropdown to show. On an upgrade of sphinxcontrib-versioning, if versions.html is refactored, we won't get the latest and greatest changes.

Would like a way to either:
A) Provide a str/tuple of strs referencing additional sphinx_rtd_theme-like themes.
B) Introspect the specified theme to figure out if it inherits from sphinx_rtd_theme.