Add an option to prevent duplicate mapping of fields. Thanks to @TornMarketing for the request.

[jonathanstegall]

What does this Pull Request do?

This changes the fieldmap screen so that if the settings are configured this way, any WordPress or Salesforce field that has been mapped is grayed out in the list of fields; it can't be mapped again. It does this on page load, and again anytime the selected options change.

If merged, this fixes #387.

How do I test this Pull Request?

• create a fieldmap, map some fields
• try to map a field that has already been mapped
• change the mapped fields
• add a new field mapping
• change the mapped field selections again