Feature: Improve the "Generate Page" shortcode attributes for a better first experience.

[kimcoleman]

Let's consider adjusting the default page shortcode that gets inserted if sites use the "Generate Page" option on the Memberships > Settings > Pages screen.

https://github.com/strangerstudios/pmpro-member-directory/blob/dev/includes/admin.php#L37-L52

We can discuss what a strong default would be.

For profile, I propose:
[pmpro_member_profile elements="avatar|256;Membership,membership_name;Member Since,membership_startdate" show_search="false"]

For directory, I propose:
[pmpro_member_directory layout="3col" elements="avatar;display_name;membership_name"]