Add MDL cmd eval, ref flex/bison integration.

Add:
	lib/mdl.y, src/engine.c
		-Delete `main()` in `mdl.y`, and redefine it in `src/engine.c`, which
		should be the engine's main entry point for a logical structure.

	src/interpreter/file_parser.(c, h)
		-Add `readMDLFile()` to read and evaluate an MDL file.

	src/interpreter/interpreter.(c, h)
		-Add `evaluateMDLScript()`, to interpret MDL commands in the global `op`
		commands array.
		-Add handlers for only several basic commands; entire MDL spec will soon
		be accounted for.