Allow stopping a running ping with a Context

[sector-f]

It could be helpful if there were a method like *Pinger.Run() that accepted a Context for cancellation. Would this be difficult to add given the current implementation?

[TheRushingWookie]

Hi I'd be interested in adding support for this. Would you guys be open to me adding a PR for it?

It'd look like

The API would look like:

func (p *Pinger) Run(ctx context.Context) error 

[TheRushingWookie]

This is implemented in #29