Add support for storing and retrieving properties based on enums

[plackemacher]

Should resolve #18. The option I went with is to provide an enum's possible values as the first argument when defining the preference variable. Another option could be to provide the enum's class itself and then using enumConstants to retrieve the list instead. That could be a bit cleaner but may require usage of kotlin-reflection which some library users might not find desirable.

[chibatching]

Thank you for your PR! 😄

I prefer to use enumConstans because it is cleaner to use.
And my bit research, enumConstants doesn't need kotlin-reflection. It's method of java.lang.Class.

Would you fix it?

[plackemacher]

No problem! 👍

I'll fix it up tonight. 😃

[plackemacher]

@chibatching updated!

[chibatching]

Merged into master with small fix, thank you!