Issue 2: Writing out in chronological order

Implementing log to save dates, expense types, and spending until
user exits program, at which everything is organized and printed
intelligently to log file.

Also, adding catch for improper expense amount input. A non-float,
non-empty string would crash the program. In use, this could be
problematic. Making it so such an error prints message to user and
asks for additional expense spending input.