Added remove-empty-groups option

[zviRosenfeldRedis]

If this option is True (default false) we'll remove groups if they have 0 processes (by default we leave the group, assuming processes from that group will be back)

The only real change is at the end of file proc/grouper.go - where I remove un-found groups if remove-dead-groups is true

fixed https://github.com/ncabatoff/process-exporter/issues/282

[ncabatoff]

This seems like a reasonable option. If you resolve the conflicts I'm open to merging it. Sorry for the long delay!

[zviRosenfeldRedis]

@ncabatoff. Thanks. I've solved the conflicts

[ncabatoff]

Thanks @zviRosenfeldRedis !