nice formatting of results

It's easier to get an overview when memory sizes and object counts are in a column of their own:
```
retained memory by gem
-----------------------------------
2106408    excon-0.39.5
58169      newrelic_rpm-3.9.1.236
2362       fog-1.23.0
1696       2.1.5/lib
481        fog-core-1.24.0
369        mime-types-1.25.1
280        activesupport-4.1.5
280        test-cloud-frontend/app
80         railties-4.1.5
```

I've also added colorized output to separate strings from file paths.