This makes a "mkdocs" command on Windows/OSX/Linux without any platform-specific code in setup.py.
I think this is the preferred way to install a Python command with setuptools.
It is, at least, the method used by:
* [flake8](8ee94d1eee/setup.py)
* [coverage.py](ca875e7390/setup.py)
* [Fabric](https://github.com/fabric/fabric/blob/master/setup.py)
* Many others
In the past, I had to change imports willy-nilly or add
"from __future__ import absolute_imports" to a bunch of files.
This is because I was renaming "mkdocs" to "mkdocs.py" instead of "main.py",
and the module-vs-script name clash was confusing imports from other files.