Implement FromStr for Map<String, Value>

[mickvangelderen]

My use case is to enable:

    #[cfg_attr(feature = "clap", arg(long))] // <- clap requires one of From<OsString>, ... , FromStr
    #[cfg_attr(feature = "serde", serde(default))]
    pub whatever: Option<serde_json::Map<String, serde_json::Value>>,

I could write a value_parser for clap:

fn parse_json_object(s: &str) -> Result< serde_json::Map<String, serde_json::Value>, String> {
    serde_json::from_str(s).map_err(|error| error.to_string())
}

But that probably isn't doing the best job with respect to error formatting.