Integration with clap 3.0

[frol]

Feature Request

Summary

clap 3.0 integrated structopts (https://github.com/clap-rs/clap#quick-example) and I wonder if it makes sense to have clap 3.0 integration in paw.

Motivation

It seems that clap is more actively maintained, so it might be better to follow it.

Guide-level explanation

It is just another option to structopt.

Reference-level explanation

The integration should be defined by an optional feature, so it will play well with the existing integrations (structopt).

Drawbacks

It is one extra integration dependency, which makes things hard to maintain.

Rationale and alternatives

We can just ignore clap 3.0 as it seems that structopt is good enough.

Unresolved Questions

Is it worth the trouble?