Do not require angle brackets in `argument`

From [ryanartecona's comment on issue #4](https://github.com/docopt/docopt.hs/pull/4#issuecomment-46645331):

> Having to specify the angle brackets in an argument lookup does feel like a mistake, though.

Thought so, too! That turned out to be trivial to fix.

If you merge this, I can open an issue so you don't forget to make a new release once you get back from your vacation. Have a good time!