Custom Key Config Option

I've created a new class and made small changes to omxplayer.cpp that allow the user to write a key configuration file and use those keybinds instead of the default by supplying command line argument --key-config <file>.

If you don't use the argument, it still uses the default key binds. In addition, if you ever want/need to change the default key bindings in the source for some reason that is now easier since they're all collected in KeyConfig::buildDefaultKeymap();

I added a section to the bottom of the README that details how the configuration file works.