Collect documentation for multiple versions of code

Note entirely sure how we'd do this, but it would be very useful to incorporate links to documentation from old versions, as can be done on ReadTheDocs. I don't think we'd want FORD to rebuild the docs for each version every time it is run. Instead we'd want it to access the old version(s) from somewhere and place them in a subdirectory. It could then offer a dropdown box on the menu bar to select which version of the docs you want to view. The most obvious problem I can foresee is getting paths to files correct. Relative paths would ensure that each version of the docs links to its own copy of each file, but it might be a bit complicated to get the links to other versions working properly, especially for the `latest` version which would presumably still continue to live in the top of the docs directory rather than within a special subdirectory.