Using number_of_values requires nightly

[SpaceManiac]

I see that structopt-derive passes along attributes to the Arg builder by default. For arguments which accept non-string values, such as number_of_values, nightly is required:

• #[structopt(..., number_of_values="1")] is a type mismatch (expected u64, found &str)
• #[structopt(..., number_of_values=1)] requires nightly and #![feature(attr_literals)]

It doesn't seem like attr_literals is on a quick path to stabilization, so some other form of support may be warranted.

[TeXitoi]

Thanks for reporting. This issue is tracked in #4.