Add boolean_add_computed_field functionality to standard add_computed_field processor

[cschloer]

RE: conversation with Adam about adding one of our custom processor functionalities to add_computed_field.

This is obviously low priority because we have the processor working for ourselves.

Code is here: https://github.com/BCODMO/bcodmo_processors/blob/master/bcodmo_processors/bcodmo_pipeline_processors/boolean_add_computed_field.py

[roll]

@cschloer
Would you be interested in proposing PRs to dataflows and datapackage-pipelines by yourself? I just feel that I can be like a broken phone here translating these changes to the main codebase. Especially for - #18 - which I don't fully understand.