Allow users to choose their desired date format in fieldmaps

[jonathanstegall]

Is your feature request related to a problem? Please describe.
Yes. Currently the plugin tries to use the default date format in WordPress whenever possible. This can cause issues though, especially with other plugins. Here's an example forum post on this.

Describe the solution you'd like
When mapping a field in the admin, if the plugin is able to detect that the field is a date field in Salesforce, we could expose a field for the date format. Users could then type in a valid PHP date format. From there, whenever the plugin tried to save data into WordPress, it would use that date format instead of the default WP format.

Describe alternatives you've considered
Currently this can be done with the object_sync_for_salesforce_push_params_modify hook, but there's no exposure in the admin for this problem.

[jonathanstegall]

giving this a start with commits 7dc14c2, f6bc613, and 1466612.

[jonathanstegall]

In WordPress 5.3, dates got a lot of improvements, as mentioned in #327. Need to make sure that whatever we do takes advantage of this.