[Enhancement] add string format support (modulo operator)

[khchen]

Sprintf or ruby's % operator for pocketlang.
Support following specifier: diuxXobfFeEgGcs. Example:

print("Number is %d" % 12)
print("Number is %d, type is %s" % [13, "cat"])
print("[%10s]" % "carrot")
print("[%-10s]" % "carrot")
print("Result is %.2f" % 12.3456)

Output:

Number is 12
Number is 13, type is cat
[    carrot]
[carrot    ]
Result is 12.35