Added idP, splitExt now looks for the last dot instead of the first.

* Added the identity pipe idP.
* Changed the behahavior of splitExt: it now splits at the last dot in
the filename instead of the first. The extension of "1.txt.bmp" is now
".bmp" instead of ".txt.bmp".
* Added the enumerate pipe in Main.hs. It gives the files names [n].ext,
[n+1].ext,... (for a user-supplied n.