Write a coverage.py extension to adapt line numbers.

[amcgregor]

According to nedbat on IRC, coverage.py version 4.0 adds the capability to write plugins. We should totally do this.

[nedbat]

Docs: http://coverage.readthedocs.org/en/coverage-4.0.3/plugins.html
An example is harder to come by. Mako is similar to cinje, but the plugin is far from done: https://bitbucket.org/ned/coverage-mako-plugin