Template tags for use in themes

[keesiemeijer]

Working on the wporg-developer theme we may have to add functions like this for use with the handbook plugin.

• function to get all handbook post types
• function to check if the current page is a handbook page
• function to get the current handbook page post type.
• function to check if a post type is a handbook post type

I imagine more themes that use the handbook could also use a standard way of getting handbook post type data.

[keesiemeijer]

First pass at template tags

[coffee2code]

In 3234:

Handbook plugin: Add four template tags relating to handbook pages.

Specifically: wporg_get_handbook_post_types(), wporg_is_handbook(), wporg_is_handbook_post_type(), and wporg_get_current_handbook().

Props keesiemeijer.
See #515, #1671.
Fixes #1693.