Remove Slop as a dependency

We don't really need the power that `Slop` is offering at this point. I think
it makes more sense to use `OptionParser` instead.

* Rename `Liftoff::OptionParser` to `Liftoff::ConfigurationParser` to avoid
 name collision
* Cleaned up the `ConfigurationParser` spec a bit
* Removed the test that just tested the contents of `liftoffrc`