Refactored various add functionality to one function. Added tests.

- 'add' and 'addm' now 'addto' with $TODO_FILE instead of user supplied file

- 'addto' now shows the capitalized filename in place of 'TODO:'.
  For example, adding to garden.txt produces a output prefixed by 'GARDEN:'
  (Probably the most controversial part of this commit and up for
  discussion. If kept, the 'listfile' output should be made to match.
  Note that this convention would then minimize standard output by
  dropping the file name.)

- All existing add/addm/add-date tests pass. Two new test sequences were
  added to test 'addto' and 'listfile' functionality.

- A space/tab nuissance was cleared up in the tests/README file.

tests/README

@@ -105,7 +105,7 @@ First digit tells the family:
 
 	0 - the absolute basics and global stuff
 	1 - basic every-day usage
-        2 - add ins
+	2 - add ins
 
 Second digit tells the particular command we are testing.