tiny helper function to splice strings
#Useage:
splice(str, index, count, add)
#Example
var splice = require('string-splice') ;
var exampleString = 'abcdefg'
splice(exampleString, 1, 2, 'ZZZ');
//'aZZZdefg'
#Attributions: Taken from the answer to this stack overflow question: http://stackoverflow.com/questions/20817618/is-there-a-splice-method-for-strings