Implement Log for Box<Log>

[lionel-faber]

Hi there! I was trying to wrap a Box<dyn Log> returned from flexi logger into async_log but I was unable to do so since the Log trait was not implemented for Box<dyn Log>.

One option was to create a wrapper struct:

struct LoggerWrapper(Box<dyn Log>);

impl Log for LoggerWrapper {
    fn enabled(&self, metadata: &Metadata) -> bool {
        self.0.enabled(metadata)
    }

    fn log(&self, record: &Record) {
        self.0.log(record)
    }

    fn flush(&self) {
        self.0.flush();
    }
}

It was suggested to me that it might be beneficial to have the implementation in the log crate so I've added the implementation in this pull request.

Would you accept this change? I will be happy to make any changes or modifications if necessary.

[sfackler]

This can be made more general to impl<T> Log for Box<T> where T: ?Sized + Log { ... }