Add checks to user input when multiple songs are found

* Added a range check, which also fails if the user enters something
  other than a number
Let the user try again if input is incorrect

* Wrapped user input and conditions in a while loop
* No longer printing to stderr or exiting for incorrect input
Play file types other than mp3

* Changed find command to use regex
* Included m4a and wav file types

NOTE: afplay doesn't seem to support ogg and flac