Provide a method to easily set log level names

[gpanders]

The add_default_keys() method sets the level field to use the short version version of log fields (e.g. ERRO for Error). If a user does not want to use the short field names, they must essentially re-implement add_default_keys() themselves.

That is,

let drain = slog_json::Json::default(std::io::stderr())

now becomes

let drain = slog_json::Json::new(std::io::stderr())
    .add_key_value(slog::o!(
        "ts" => slog::FnValue(move |_: &slog::Record| {
            time::OffsetDateTime::now_utc()
            .format(&time::format_description::well_known::Rfc3339)
            .ok()
        }),
        "level" => slog::FnValue(move |rinfo: &slog::Record| {
            rinfo.level().as_str()
        }),
        "msg" => slog::PushFnValue(move |record: &slog::Record, ser| {
            ser.emit(record.msg())
        }),
    ))
    .build();

which also requires adding a (top level) dependency on the time crate.

I would like to be able to use the default keys, but be able to choose the strings used for the log levels. Example API:

let drain = slot_json::Json::new(std::io::stderr())
    .add_default_keys()
    .with_log_levels(["off", "critical", "error", "warn", "info", "debug", "trace"])
    .build();