Generic command documentation

[AutonomicPerfectionist]

This PR adds functionality to document any CMake command, whether or not they are a known definition command. For example, if the following is in a file called example.cmake:

#[[[
# This is a documented message command
#]]
message("This is a message")

Running this through CMinx will generate the following RST:

#############
example.cmake
#############

.. module:: example.cmake

.. function:: message("This is a message")

   .. warning:: This is a generic command invocation. It is not a function or macro definition.

   This is a documented message command

This fixes #51

[AutonomicPerfectionist]

@ryanmrichard does this look like what you were thinking or should the admonition be adjusted a bit more?

[dschiller]

Amazing Branden and Ryan!!