Ability to load shader on launch from command line

[danaYatsuta]

Using "--shader" option followed by shader folder, that shader can be loaded on launch.

My personal reason for implementing this was to launch some games with gba-color.shader and some not depending on whether the game oversaturates colors to counter original GBA's dark screen or not, but it may have other uses.

[endrift]

Since the SDL fronted doesn't support shaders yet, I should decide if I want to add support for that before merging this, or ask you to move the command line setting to the Qt subparser. The former probably makes more sense.