Add app providers, tests, docs

docs/glossary.rst

@@ -42,6 +42,10 @@ Glossary
 
      See also the human-friendly :term:`app title`.
 
+   app provider
+     A :term:`provider` which pertains to the :term:`app handler`.
+     See :doc:`narr/providers/app`.
+
    app title
      Human-friendly name for the :term:`app` (e.g. "Wutta Poser").
 
@@ -104,6 +108,11 @@ Glossary
      modules etc. which is installed via ``pip``.  See also
      :doc:`narr/install/pkg`.
 
+   provider
+     Python object which "provides" extra functionality to some
+     portion of the :term:`app`.  Similar to a "plugin" concept; see
+     :doc:`narr/providers/index`.
+
    settings table
      Table in the :term:`app database` which is used to store
      :term:`config settings<config setting>`.  See also