Allow template for index.html to be in source branch

[goerz]

In line with the design goal of keeping all configuration for doctr-versions-menu on the source branch (#6, #7, #9), it might be good to have some way for the index.html_t template file for customizing index.html live on master.

Fundamentally, this is tricky, because it is not trivial to move files between branches during the deploy. However, the resolution to #6 showed that environment variables are a good way to transfer data outside of the file system. Maybe, in addition to looking for a file index.html_t on gh-pages, doctr-versions-menu could also check whether an environment variable DOCTR_VERSIONS_MENU_INDEX_HTML_TEMPLATE is defined that contains the full template. Then, index.html_t could be stored on the master branch (e.g. in docs/_templates alongside the other Sphinx templates), and .travis.yml could read the contents of that file into an environment variable.