Add templating that covers github release page features

[wagoodman]

Change the user template for the changelogs generated for github to use @user instead of [user](profile) to get formatted results in the releases page that annotates individual contributors:

There are probably more features to look at here, this is just an example. To cover these kinds of things it would be nice to be able to specify a name for a format that is tailored for this kind of output:

$ chronicle ... -o github-release

# get markdown that is targeted for github release pages