Add mode to ingest pre-prepared coverage files

[Dave Brophy]

@haya14busa:

Why not accepting coverage report text files instead of wrapping go test and executing tests?
I like the concept but I don't want to replace go command as much as possible.

[Dave Brophy]

I didn't add this because it's not quite as straightforward as you might think:

1. Unfortunately go test can't accept /... to indicate recursive packages when combined with code coverage:

$ go test -coverprofile=coverage.out ./...
cannot use test profile flag with multiple packages

... so the courtney command has to manually scan the filesystem for sub packages and run one go test command for each package.

2. In order to get coverage logged cross-package, you need to add all the packages you're testing as a comma separated list to the coverpkg argument.

You can see the go test commands that we're running when you add the -v flag.

However, I can see that this might be of use in certain situations, so I'll look into adding it as an option.

[Dave Brophy]

I've added a -l option that allows you to skip the test step and load pre-prepared coverage files. Let me know what you think: 715958a

[haya14busa]

the problem is related with golang/go#6909 and I fixed it by creating https://github.com/haya14busa/goverage