Use serde to serialize-deserialize configs, including yaml

[CreepySkeleton]

Summary

Implement serde::Serialize/Deserialize for clap::App so it could be deserialized not only from a YAML file, but from any other format.

Motivation

Currently, yaml supports deserializing only from YAML via App::from_yaml. This restricts users to only the one blessed format, while the layout of clap::App is pretty simple on it's own and can be expressed in many config formats, such as JSON, TOML, .plist to name a few.

Supporting only the chosen one format is kind of inane nowadays, when we have serde, which handles the differences between formats in a generic way.

Not to mention a number of "YAML is too WTF-full format for config files" considerations. Some users (myself included) would happily move to TOML if it was supported out-of-box.

User experience

As for users of clap who only interested in YAML, nothing would change. From "user experience" side, there's no actual difference between

let yml = load_yaml!("file.yml");
let app = App::from_yaml(&yml);

and

let config = std::fs::File::open("file.yml").expect("No file.yaml!");
let app: App = serde_yaml::from_reader(config).unwrap();

But users interested on other formats, like TOML, JSON, you name it, could really make use of this feature.

Implementation

I gave a quick look-over to App and Arg layout and I'm fairly certain that Serialize and Deserialize can be simply derived.

Related issues

#918: Serializing to / deserializing from JSON. @kbknapp agreed this feature would be pretty useful.

#853: use serde_yaml since it provides much more useful error messages.

[jonericcook]

Would this feature allow for the following behavior?

./examplecli helpdump --json helpdump.json

and/or

./examplecli helpdump --stdout

The helpdump.json and stdout would contain all the information about how to use examplecli, essentially the --help for each command and subcommand with all the descriptions and arg type input.

The reason I ask because I have a handful of rust binaries that use clap and i would like to be able to call, for example, the helpdump command to get a json object that tells me how to interact with the binary so I can programmatically use the binary from another language.

I guess in a nut shell i'm asking for a rust program that uses clap to be able to output a .json file and/or a json string to stdout that describes how to use it so that it (the rust program) can be used by another program just by ingesting the .json/json string.

[pickfire]

Maybe we could also merge configuration with the commands, with the configuration in file overwritten by the configuration specified by command arguments?

[throwaway1037]

I think RON should be supported.

Also, I think DSL support should only cost compile time and be baked into the binary, with no cost at runtime since it's essentially a declarative specification of the command-line arguments for the program. The alternative is reading the file at runtime, and then having to distribute it with the program, which is slower and uses more storage space. I don't know which is the current behaviour.

[epage]

We have decided to deprecate the YAML API. For more details, see #3087. If there is interest in a YAML API, it will likely be broken out into a clap_yaml and developed independent from the core of clap.