Extract tags from github

[andreww]

Github now includes a lightweight "tagging" system so that authors can indicate the areas where the software can be used. We should extract these using the github API. This could be useful for us to see if software designed for one research area is being used by another. I suspect a more likely outcome is that we'll discover how quickly (or not) people have adopted github tags.

[andreww]

I don't mean tags (although we can get the git tags with repo.get_tags(), and these could be a useful indicator of maintained software) but topics. Topics are "currently available for developers to preview" (https://developer.github.com/v3/repos/#list-all-topics-for-a-repository) and not in pygithub.