Include ability to define "donor role"

[tmmbecker]

In #825, we added the ability to segment by user login status and defined a "user" as:

for logged-in WP users who are subscribers or woo customers only (not admins, editors, etc).

For those who run donations offsite, this presents a problem as they aren't able to define those donors as "users" and thus their opportunities for segmentation are more limited.

If we allow the publisher to define the roles that match a "user," that would allow folks to hook into the WordPress system and still define a donor that can be used for segmentation purposes.

Requested by El Libero.