Attempt at Supporting Overriding Core Delimiter Processors

[rdestefa]

Quick attempt at supporting #386. Not sure if I've covered all edge cases or if there's a simpler way to do this, so might come back to this later.

[robinst]

Instead of a separate configuration, can you explore how it would look if the normal customDelimiterProcessor would just be able to override built-in ones? I think we'd still want to error if a user accidentally specifies two processors with the same configuration, but not if they override a built-in one (i.e. add the custom ones first, then add the built-in ones if they don't have conflicts).

[robinst]

See comment above