Implement with for read access to MDC

[nwtnni]

Addresses #1.

It would be fairly straightforward to implement a symmetric with_mut function as well:

pub fn with_mut<F, T>(f: F) -> T
    where F: FnOnce(&mut HashMap<String, String>) -> T
{
    MDC.with(|m| f(&mut *m.borrow_mut()))
}

I don't have an immediate use-case for this, though. I can add this in if you'd like, although it feels a little too powerful for the purposes of MDC.