X-diff-cover

* Fix a bug in which function names and TODO: were not appearing in
`diff-quality` reports.

* Add support for code quality metrics (pep8 and pylint for now).
Invoked via call to `diff-quality`

* Add support for multiple coverage XML inputs.

* Refactored diff report generator to use Jinja2 templates

* Fix bug in which adding then deleting lines would result in
incorrect lines included in the diff report.

* Fix a bug in which plus signs in the hunk line would cause a parser error

* Fix bug in which lines around a change were included in the report

* Compare with origin/master instead of master
* Includes staged and unstaged changes in report