Implement reporter pattern

[shellscape]

Cheers for the plugin. I'd like to request implementation of a reporter pattern (that you'll find on most linters, test runners, etc) so that we can choose how to display the results of the bless operation on our files, rather than having a predefined message output through gutil.log.

Typically this is done by adding a property such like bless to each file in the original stream, with some useful properties attached.

[adam-lynch]

Sounds interesting. I assume the default reporter would be what we have now