Allow visibility to be applied to context selectors about snafu

[shepmaster]

I have a partially-implemented branch where I quickly hacked up the default to be pub(crate) — if you are experiencing issues, you can try that and see if it helps your case.

[mikeando]

I think this solution will work for me.

I initially wondered about the multiple #[snafu::xyz] bits that could be added to each entry,
and whether they should be combined into a single attribute. i.e. instead of

#[derive(Snafu)]
enum Error {
    #[snafu::visibility(pub)]
    #[snafu::description("a:{}", "source")]
    A{source: io::Error},
    ...
}

it could be something like

#[derive(Snafu)]
enum Error {
    #[snafu(visibility=pub, description=("a:{}","source")]
    A{source: io::Error}

But thinking deeper about it - the slightly more verbose first version may be better handled by source control, so I think it is better.