`set` builtin not completely implemented

The set builtin command is not fully implemented. The -x and -o flags are still yet to be done.

However, implementing them might need some fixes up in the higher level. As I looked into the way that the keybindings are set through the command line flags here and here, the keybindings are set in the context field of the InteractiveShell type. But the set builtin (and probably other ones as well) are only passed the Shell type only. I haven't dug further, but I suspect that a Context isn't always tied to a Shell, and we might need to move some of the fields around.