Add support for literals in arithmetic

[Taeber Rapczak]

XPOS	EQU 1
YPOS	EQU 0
	LDA SHIPS+XPOS,X
	RTS
SHIPS	DFB 00,00

Should assemble to BD 04 80 60 00 00, but fails with error:

expected hex, binary, or decimal literal; got XPOS