Re-factor StyleGuide for ease of type addition

This change makes it easier to understand `StyleGuide::Base` inheritance. The template method pattern makes it easy to scale style guides to scale horizontally: just add the new language style guide and inherit off of `Base`.

Changes:
* promote `Line` to a class
* add `Violations` object to manage unique violations
* refer to modified lines as changed to make it easier to reason about

What do you think about returning a null object here? I know you don't like Null in the name, but it is clear. Maybe `EmptyLine` or `NullLine`?