Add support for guest_group field

[mattstratton]

Fixes #159
Fixes #137

This change adds a new field to guest pages, guest_group. Now, if you set multiple guests to the same guest_group value, they will act in the following way:

1. A guest that is in a guest group will list all episodes for all guests in that group on their individual page
2. If guests are in a guest group, only the most recent (as set in frontmatter date field) will display on the guest list page