MGCB Improvements

A bunch of improvements to the MonoGame Content Builder.

 - Removed .contentproj support.
 - Refactor of command line parser.
 - Support for "response files".
 - Preliminary support for cleaning output and intermediate directories.
 - Fixes to PixelBitmapContent where it was using height instead of width.

I also put up some [preliminary documentation](https://github.com/mono/MonoGame/wiki/MonoGame-Content-Builder) for using MGCB.