Is a impure library `re` used ?

[jiro]

I think that docopt depends a regex library, not re.
regex library is library in pure Nim.
I think that you can delete a below line of README.md.

An impure `re` library is used.

https://github.com/docopt/docopt.nim/blame/master/README.md#L139

[Oleh Prypin]

Needs followup after #42