Adding support for inner anonymous classes

Closes #27 
I actually just started extracting a `NamedTypeWriter` subclass out of `TypeWriter` (not having even read your comment yet – great minds think alike ;-) – I was coming here to reply to your first comment about the re-use of `TypeWriter`) and then realized that we also inherited modifiers and annotations from the `Modifiable` superclass. So yes, I think we need `AnonymousClassWriter` as a peer of `TypeWriter` without hierarchical relationship.

We might want to introduce some kind of `ClassBodyWriter` in the future to also support [enum constants with a class body](https://docs.oracle.com/javase/specs/jls/se7/html/jls-8.html#jls-8.9.1) and use it as a delegate everywhere a class body is expected (class, interface, enum, anonymous class, enum constant). Or maybe this is what we should do here, and leave the `new Type(ctorArg)` part of anonymous inner classes to the `Snippet`; e.g. `Snipper.format("new %s(foo, bar) %s", typeName, anonymousClassBody);`