added sort case when numbers having digit at the end and rspecs

I have added a feature in which this gem will be able to sort the array of strings in way that if digits appear after alphabets in an element then the elements will be sorted numerically.

e.g. ["c1", "c10b", "c11a", "c2"]  #=> ["c1", "c2", "c11a", "c10b"]


Resolved issue #2