Allow for definition of AJAX action slugs and adding actions in AjaxHandler\AbstractBase

[sdunham]

Right now, in order to actually add actions to use any custom AjaxHandler classes, you have to call add_action one or more times in Site::configure() (or functions.php if you're lame 👎) with whatever action slug(s) you've chosen to handle. I think it would make more sense to at least allow for the option of registering the action slugs you want to handle in your AjaxHandler class and provide a function to call in the config callback which will make the appropriate add_action calls based on those slugs.

Expected behavior

To register an action to be handled by a class, something like:
MyAjaxHandler::register_action($slug, $includePriv = true, $includeNoPriv = false)

To add_action(s) in the config callback based on previously-registered action slugs, something like:
MyAjaxHandler::add_actions()

Actual behavior

Right now, action slugs don't exist in AjaxHandler classes unless you're handling multiple actions with a single class, and even then you have to make individual calls to add_action for each of those slugs.

Steps to reproduce behavior

See Conifer\AjaxHandler\AbstractBase

What version of WordPress/PHP/Timber/Conifer are you using?

Conifer v0.3.0

How did you install Conifer? (for example, via git clone, GitHub zip download, Composer/Packagist?)

Cloned from GitHub

[acobster]

@sdunham did #66 address this fully? If not, can you update the description with Acceptance Crtieria?