Adds support for passing usernames into rake task

Now you can pass usernames into the digest task. I did this because I often pair
and want to see what my pair and I have completed in the digest. But I also
often switch pairs, so editing the .env file every day can be cumbersome.

Now you can run it like this:
`foreman run bundle exec rake digest["justinxreese marktfrey"]`

If you're using zsh like I do, you have to escape brackets in args :/
`foreman run bundle exec rake digest\["justinxreese"\]`

Unfortunately, it looks like commas break rake arguments so the params have to
be separated by spaces.