Add interpreter commands documentation.

Add:
	doc/interpreter.txt
		-add documentation for the interpreter's commands, providing the command
		character, arguments, and a description for each.
Add shell.txt, content to interpreter.txt.

Add:
	doc/shell.txt
		-add documentation for the engine's interactive shell, interacting
		recognized commands and keyboard events.

	doc/interpreter.txt
		-add section explaining the format of each documentation command
		-add hanging indents to all command "Description" sections greater than
		one line in length.
Add script_file.txt, ignored lines to interpreter.

Add:
	doc/script_file.txt
		-add documentation for the engine's script files.

	doc/interpreter.txt
		-add paragraph specifying ignored empty lines and comments.
Add project structure documentation.

Add:
	doc/graphics_engine.txt
		-add documentation for the project's directory structure -- more content
		to be added later (eg, information about command line options, the
		shell).