Improvements to command line interface

[KantarBruceAdams]

Re: #23 (comment)
"It looks a bit strange to have these options interleaved with -test, but I didn't see any better way to avoid all the quoting and escaping issues when passing command line arguments."

May I suggest using -- with the standard unix convention to mean end of options.
So that:

gobco -test -test.v -verbose -test -test.count -test 5 -test -check.vv

Would become

gobco -verbose -- -test.v -test.count 5 -check.vv

That is you are required to give gobco options before options to go test

The -keep option is less useful without a way to set the location of the temporary directory.
I would like to suggest a -workspace option or similar.

I note a gradual build up of gobco directories in /tmp (as per #9 ). I am not using the -keep option but using a new -workspace option I could guarantee my build doesn't cause this.
I'm not sure yet if these are produced when the tests fail or when I press ctrl-C to abort them or both or something else.
You possibly need to add a signal handler to clean things up before the program exits.