Support cleaning up unused imports

[mjpitz]

Right now, I find myself needing both this tool and goimports. As far as I can tell, the only thing goimports does that go-groups does not is clean up unused imports. It would be nice if go-groups supported this as it would drop the need for goimports almost entirely.

[cnmcavoy]

Note for myself, the actual goimports logic to remove imports is here: https://github.com/golang/tools/blob/master/internal/imports/fix.go#L400

It looks like go-groups would have to make the jump from string parsing to ast parsing the input source files to achieve this. I'll definitely look into this, but it's not a trivial improvement.