add a flag to ignore dev-dependencies

[jyn514]

i use cargo-outdated to see which of the crates in my final binary will end up outdated. i don't really care if my dev-dependencies are outdated. i'd love a flag that says "exclude dev-dependencies", the way cargo add has a --dev flag.

[davidhewitt]

It looks like the metadata would make this possible, a PR welcome to implement this flag.