Feature Request: weakdeps

[schlichtanders]

The package extensions for Julia Project.toml are really cool and I use them a lot.

It would be great if CondaPkg.toml could also have a similar section, so that package extensions can also define Conda dependencies. Not sure how easy or impossible this is. Would be helpful.

[cjdoris]

Can you give an example CondaPkg.toml and describe the behaviour you'd like? It's not obvious to me.