Use Handlebars to provide more flexible and robust templating.

Fixed order within sample output in readme.
Extend partials separately from the options method so it's not all or nothing when providing custom partials.
Fixed a small error in the readme.
Optionally pass after and before as arguments. Fixes #8.
Merge pull request #8 from creynders/use_args

Optionally pass after and before as arguments. Fixes #8.
Added the ability to pass a commit SHA or tag for the after and before options. Closes #3
Added more examples to README for specifying after and before options.