Fixes, more features, and new Functions (v. 0.3)

-Improved WordFrequency function, although it still crashes
-Improved CorrectCapitalization function to correct mistakes after '!' and '?'
-Added CaesarShift function, which still needs more work
-Added new feature: last file used is saved, so user doesn't have to re-enter file name
-Begun adding option for user to enter text manually
-Added helper function: isLetter()