Add TryParseVector to StringUtil.
Adds a nice way to have options be in a vector for results
Add the VideoCommon PostProcessing class.

This class loads all the common PP shader configuration options and passes those options through to a inherited class that OpenGL or D3D will have.
Makes it so all the common code for PP shaders is in VideoCommon instead of duplicating the code across each backend.
Add the PostProcessing class object to RenderBase in VideoCommon.
Backends will initialize this variable with their own inherited PostProcessing class object.
Change OpenGL's post processing to use the new VideoCommon PP object.

Let's OpenGL's PostProcessing namespace be changed to a class inheriting from VideoCommon's PostProcessing class.
Change all the post processing shaders to the new system.
Removes the README.txt file in favour of a new wiki page I'm going to generate.
Add the configuration dialog for post processing configuration options.
Only enables the config button when the shader options available to it
Merge pull request #693 from Sonicadvance1/PP-shaders-improvement

Improved system for post processing shaders.