Add new validation feature for settings.

[jwoertink]

Fixes #41

This adds in a new feature that lets us add some validation beyond just the existence of a value to our settings. When you add the validation key, you specify a symbol of the method that will run the validation. This will take an argument of the type you specified in your setting.

Inside that method, you can run any sort of validation you wish. If that validation fails, you can call settings.invalid() with a message to be raised. This will raise an InvalidSettingFormatError with your message.

[paulcsmith]

This is a really nice implementation. I thought it'd have to be more complex but you did it in a super simple and powerful way. I dig it.

Just one comment on making it clearer that it is raising, and where the method comes from (Habitat)

[jwoertink]

@paulcsmith If you're cool with this, I'll merge it in. 👍

[paulcsmith]

Looks great :D