Making file extension optional, setting .sls as default for salt

Hey there!

Holidays, weekend, so why not dip my toes in reclass and add support for a custom file extension.
It's cosmetic, I know, but it will make it easier to convince my friends to use reclass ;)

NOT TESTED FOR ANSIBLE!!!

I don't use it (yet) so don't have anything setup to test if it works for ansible (so please pull this down and see for yourself)

Signed-off-by: Koen Schmeets <k.schmeets@gmail.com>