Feature Request: Being able to print a summary of triggered checks

When applying ErrorProne to an existing project you might end up with lots of warnings, even more than javac would print to console (defaults to 100).

I think it would be nice to have an option that tells ErrorProne to output a summary table which shows all triggered checks order by occurrences, e.g.

```
ERRORS:
-------
12x [AlwaysThrows] -

WARNINGS:
---------
200x [EqualsGetClass] -
12x [JdkObsolete] -
```

Then one can easily see an overview, can read more about the different bug patterns and you have a good starting point to improve the code base and/or configure ErrorProne. Obviously one would fix the errors first, but especially for warnings I would prefer incrementally work on a topic one by one. So to reduce the noise during compilation I would disable all warnings and then enable them one by one based on the initial overview output.