RFC: Add support for custom input formats

This adds an InputFormatProxy along the same lines as the other proxies.

Unfortunately, I couldn’t really figure out how to write a meaningful unit test, so I implemented a (rather contrived) custom input format in the integration tests to at least make sure the code runs.

I needed to make a few changes to the Rubydoop core in order to get this running. 
 - Setting the job script name earlier to allow dispatching to proxy classes from the configuration DSL. Good?
 - Changing the create_* factory methods to allow getting at the class as well as an instance. Bad?

Personally, I would really have preferred a `#subclass_of?` or something along those lines, but it seems using comparisons is what you "should" do in Ruby