Rename sig_handler(), add command-line argument.

Add:
	src/engine.c
		-if any command-line arguments are given, the graphics_engine interprets
		the first as the path/filename of a script file, and will (attempt to)
		evaluate it.
		-otherwise, it'll run a hard-coded test*() function (currently
		testMatrix()).

	src/utils.c
		-rename sig_handler() to sigHandler(), to conform to camel- over
		snake-case.

	scripts/scripts.gsc
		-finalize script contents, add a command to write the final screen to
		a file; insert some newlines/comment out several commands, to
		demonstrate parsing robustness.

	src/parser.c
		-now ignores lines beginning with both newlines and comment chars.