Make ThemeManager an abstract class and extend with different datastore classes

[ericdowell]

Summary

Reading directly from the filesystem is fine, but sometimes it's fast to read from a database or cache store (redis, memcached, etc).

Technical Summary

Make ThemeManager a base abstract class that gets extended by specific datastore classes. Also add a key to the package config file that can control what datastore will be used when processing the themes.

Initial datastore classes will be:

• File
• Model Database (Laravel specific)
• Cache Store (Laravel specific)