Integrate MDL lexer/parser.

Add:
	lib/
		-Add dir for libraries.

		lib/*
			-Add all files necessary for MDL parsing.

	makefile
		-Add rules to compile the lexer/parser with `flex` and `bison`, and to
		compile the new object files into the engine.

	src/engine.c
		-Replace `main()` with `engineDriver()`, as the executable's `main()`
		now resides in `lib/mdl.y`/`bin/y.tab.c`.