Allow for alternate annotator classes

[braswent]

Allow Dug Annotator to be defined by the user by following the same protocols used for the Parser on Annotators.

The following functionality is expected:

• Pass the annotator Class name as a string to a Pluggy hookspec function
• Define the existing Annotator class in the same way that the Parser class is defined in core/parsers/__init__.py
• @YaphetKG should we do this too? Connect the Annotator class selection into the Roger repo in dug_helpers/dug_utils.py and metadata.yaml

[YaphetKG]

Yeah but i think we can do that after we wrap this up and integrate it in a later version of roger maybe?