Added the substraction to forth interpreter

Added new function "-" to the forth interpreter.
Also added tests in cucumber test to test this new feature.
To use : try to type "4 5 - ." in interpreter
Added multiplication to forth interpeter.

Added the "*" in interpreter.
Added test for this operand as well in cucumber.
To use just type "5 6 * ." in interpreter.
Fixed typo in multiplication test