subscriber: Add MakeWriter impl for Option

[joshka]

This adds a MakeWriter impl for Option<M> where M: MakeWriter.
This makes it possible to use Option<MakeWriter> in combination with
other MakeWriter implementations (e.g. Tee).

An example is found in #776,
where depending on the configuration, the user might want to use any or
all of File, Stdout, and Stderr writers.

let stdout = enable_stdout.then_some(std::io::stdout);
let stderr = enable_stderr.then_some(std::io::stderr);
let file = path.map(|f| File::create(f).ok());

let multi_make_writer = stdout.and(stderr).and(file);

Motivation

I was looking to contribute by trying to close out old PRs and tasks. The current oldest is #776. In that, @SriRamanujam expressed a need where they could combine multiple make writers easily. This is currently tricky as the types would have to be type erased in order for this to work. Making Option<impl MakeWriter> implement MakeWriter simplifies that significantly.

Solution

Implement MakeWriter for Option<M> where M: MakeWriter