Allow specifying authors

[evandcoleman]

This allows the user to limit stats to specific email addresses to decrease the time it takes to generate stats.

[tomgi]

Hi @edc1591, thank you for submitting that improvement!

I think you should add this new optional parameter to https://github.com/tomgi/git_stats/blob/master/lib/git_stats/cli.rb to allow the list of emails to be provided when invoking the app.

Could you also add some tests for this new behavior?

Thanks