Code-reviewed by Nate; notes follow.

CHANGES AND LEARNING:
- Newline definitions were unnecessary and bad ideas
- My file handling was sloppy and unnecessary --> IOError
- Was using ArgParser a bit incorrectly
	- no need for separate -a and -s
	- is better suited to also be handling the location of
	  my files, allowing for user-defined paths
- Learned that it's a terrible idea to prepend to files; if a
  file is fragmented, prepending requires the loading of the
  first chunk, pushing everything out, then finding the next
  chunk to insert the displaced line from the prepending,
  which in turn will load (and push out) the next chunk, and so
  on until it gets to the end of the file. Appending only
  requires fiddling with the final piece, so we fixed that.
- In that vein, it was then silly to keep the function which
  returned the first line of the file.
- It's silly to name a variable 'xyz_str' when its name has no
  bearing on its type.
- Python can Pickle objects to store them as non-human-readable
  file, then unpickle them to restore the object back to pre-
  pickled status. Pretty cool.
- Holy crap, such a cool thing with multiplying by 1 or -1.

OTHER THINGS TO DO:
- Possibly remove the subtraction arg entirely
- Make totals message display after adding/subtracting
- Look into possibly using a database to store things like
  diff/timestamp/reason (SQLite)