Expose relationship start/end dates in relationship processor

[Upperholme]

Is your feature request related to a problem? Please describe.
I would like to be able to collect data about start and end dates of relationships. For example, I'm using a form to collect information about counsellors. This includes information about who the counsellor's supervisor is. If the counsellor changes who their supervisor is, it is important to know when the relationship ended with the old supervisor, and when the new relationship began.

Describe the solution you'd like
In the relationship processor, make the date fields available as provided by CiviCRM so that the magic tag for a date field can be mapped.

[kirk-circle]

Hi @mecachisenros, I also need this feature asap. I'm going to try to submit a PR for it today (against the dev branch). Have you done any work or got any pointers for me before I get started?

[kirk-circle]

Raised PR #174.