DRY config a little further

we could DRY the config a little further to go from:

``` rb
ProjectMetrics.configure do
add_metric :code_climate_project_metrics
add_metric :github_project_metrics
end
```

to

``` rb
ProjectMetrics.configure do
add_metric :code_climate
add_metric :github
end
```

which allows less repeated text in the config component - and would also simplify the code for returning of the list of configured metrics