adds SubscribeAsyncWithNewContext

[szimmer1]

Adds and implements an interface method SubscribeAsyncWithNewContext with same signature as SubscribeAsync but is "Context-aware" by creating a new context.Background() to pass to the handler function.

My company has this use case because our server handles incoming requests partly by Publish-ing an event to a topic with at least one async subscriber which makes other outbound network requests. The server cancels the context once it sends a response. Routinely, the cancelled context from the upstream request prevents the subscriber handler function from completing.

This feature allows subscribers to opt into being called with a background Context so that cancelled contexts do not adversely affect them.

[szimmer1]

Looks like this project supports go versions well before 1.7 when the context package was added to the standard lib. Feel free to close the PR if this is a breaking change.