Link to source - sample implementation

Hopefully at least some of this code is useful. See #63

By passing source_url and source_dir to the Sami config, with this PR you can get links to view source for Github. For example, if your source files are in myproject/src, you'd pass myproject (omitting src) as the source_dir and github.com/myuser/myproject as source_url.

For github it'll use tree/<version> for the url - this probably isn't very flexible but not sure how it can be improved quite yet.