Add support for tracing-subscriber

[ahadnagy]

This PR provides a tracing_subscriber::Layer implementation for recording log events in applications that use the tracing crate for logging.

[gin66]

Thanks for the PR. If I understand this correctly, this allows messages from the tracing crate use tui-logger's features.

From my point of view looks good, but I think this feature is better an opt-in.
And actually the slog should be opt-in, too. But I may change this in the next version.

Could you please update ?

[ahadnagy]

Thanks for the review!
I made the feature optional.