Add Setting for Map Index-ability

[morehawes]

Waymark uses the custom post type waymark_map to store Map data. Currently Maps are public and indexable by search engines (see this support topic).

Ideally this would be controlled by a Setting, so users can prevent Maps from being indexed.

Currently this is hard-coded using register_post_type in inc/Waymark_Types.php.

A work-around is to block access via robots.txt:

User-agent: *
Disallow: /map/*