Fixed a trivial indentation issue in Mittens.Slug

@gibiansky's hindent style messed up the indentation slightly. That bug is now
fixed.
Switch Mittens.Slug from String to Text.

Previously, Mittens.Slug was a wrapper around String. I did this primarily to
make the code simpler, initially. However, Data.Text is faster than String, and
it doesn't make the code that much more complex. All of the tests pass, so this
is a go.