Add 'rate' command

After this morning's rate failure, I was looking for an easy way to see the current limit and when it would be reset. Easiest seemed to be to add a `scc rate` command with output:
```
  Core: 4976 remaining from 5000. Reset at 09:05
Search:   30 remaining from   30. Reset at 08:05
```
If this is useful, happy to add the tests, etc. Also, would a `--require=100` be useful in order to wait for the reset time if 100 are not available? This command may should also be run before and after every build so we see how many tokens were used. (Or before/after each top-level command?)