Support custom field in transfer_map with property name

[rpbaltazar]

as part of the transfer_map, there should be an attribute for custom that supports a key to store the values in user properties.

e.g.
CSV File headers:
Name, Family Name, Favorite color

the transfer_map would be:
name -> first_name
family name -> last_name
favorite color -> custom, key: 'color'

This would then create a user with the properties: { color: }