Support for special "Development Dependencies" section

[mcandre]

NPM and RubyGems support "developer dependencies", for things like linters and transpilers, that are not necessary at application runtime, but are helpful for developing the app.

This turns out to be really helpful for:

• Clarifying dev tool dependencies, so the user isn't left guessing how to setup a project's dev environment. We're no longer in the '80s, where a random project online could be expected to build with just gcc/make/sh.
• Automating dev tool setup, so the user doesn't have to perform a lot of manual dev tool install steps. This is often haphazardly managed by shell scripts.
• Optionally pinning and locking dev tool versions, as a defensive measure against dev tool interface breakage. Some linters break their configuration syntax, and semver is a bad joke.

Could dep get a similar feature?

[jstemmer]

There's some discussion around this in #221

[sdboyer]

Quite reasonable requests - and thanks @jstemmer, you beat me to it :) Yup, I'd say that issue covers everything here, so marking this as a duplicate.