This essentially implements the closest we can get to PEP 562 which allows for modules to control `__dir__` and `__getattr__` in order to deprecate attributes. Here we provide a wrapper class for the module in `util`. If a module has attributes that need to deprecated, we derive from the wrapper class and define the attributes as functions with the `property` decorator and the provided `deprecated` decorator. The class is instantiated with the module's `__name__` attribute and the class will properly replace the module with the wrapped module.  When accessing the depracted attributes, a warning is raised. Closes #739.

New `toc_tokens` attribute on Markdown class.
Contains the raw tokens used to build the Table of Contents. Users can 
use this to build their own custom Table of Contents rather than needing 
to parse the HTML available on the `toc` attribute of the Markdown 
class.

And other docs cleanup.