Doc feature flag to document all features

[SuperFluffy]

When writing the doc string in #910 I noticed that while I reference App::from_yaml, it is not contained in the resulting documentation. Only when passing invoking cargo doc --feature yaml will cargo also build the documentation for that method.

To fix this and similar feature-related issues, we could include a feature doc or all which contains all the possible features.

I am not sure how we can make docs.rs pick the appropriate flags though.

[kbknapp]

I'd like this. But I'm also not sure how to provide flags to docs.rs. Pinging @onur

[onur]

I'll finalize onur/docs.rs#73 soon.