Add string subtract/multiply/mod support

 - f_minus(string1, string2)
        -> if string1 ends in string2 return string1 without the string2
           suffix

 - f_multiply(string, number)
        -> string + string + .. + string -- number times

 - f_mod(string1, string2)
        -> if string1 states with string2 return the remained of string1
           after removing the string2 prefix

Still missing: f_div() support.
Test new string operations