Add StringUtils#unwrap

Proposal to add a new method namely unwrap to StringUtils.

The method will be responsible for unwrapping a string token from a character (or another string token). Basically, It will be the opposite of the StringUtils.wrap() method.

Possible outcomes of the method

     StringUtils.unwrap("\'abc\'", "\'")    = "abc"
     StringUtils.unwrap("\"abc\"", "\"")    = "abc"
     StringUtils.unwrap("AABabcBAA", "AA")  = "BabcB"