Storing partial context on the context manager

Adding any new partial data to the context manager when it's loaded so a reference is setup.
Caching context for custom templates

Adding a context level for new template types and caching their data when added.
Use `.template` to load deafult template types

Adding `defaultTemplates` to load `pages`, `partials`, and `layouts`.
Removing the methods specific for loading pages, partials and layouts and moving their documentation to `docs/api-default-templates.md`
I think this will be a good idea for any defaults that we're settings, then we can include the doc in the comments for the `default*` method.
Adding layoutDelims to options for default engines

Doing this for now.