plz cover integration

[HartBlanc]

Please provides coverage information via the cover command.

This command is useful but has an interface where you have to pass the paths of the files to include coverage for and the test to run, which is a bit laborious to do manually.

Having a tighter integration with this plugin could make it much easier to use by utilising the context of the currently open buffers and the existing functionality for running tests.

An example of how this could be integrated would be to provide a cover command which runs a test (approximately in the same way that we can run tests with the test command today.) which then annotates coverage information (e.g. by colour coding the line numbers) for all open buffers.