Fixes #8.

`--` will now work as intended, regardless of whether `options_first`
is enabled.
Bump version to 0.6.2.

Removed the Cargo lock file for `docopt_macros`. I'm not sure
how to keep it properly updated. (It seems there is a chicken
and egg problem since both `docopt` and `docopt_macros` are in
the same repo.)