Add Outbreak Threshold function

[joshwlambert]

The paper "Introducing the Outbreak Threshold in Epidemiology" describes a simple metric that can be used to determine outbreaks or transmission extinction. This could act as a useful function alongside probability_epidemic() and probability_extinct().

[chartgerink]

Is it in relation to this paper in PLOS Pathogens?

It would be helpful to write the metric out here, if still relevant to the package, to make the issue more self-contained and encourage new contributors to try it out :)