svc: support closing via context

[jehiah]

In nsqio/nsq#1305 I encountered a situation where I wanted to add additional hooks for exiting a process - but the only exit path for Run() is a SIGINT/SIGTERM. In order to add new way for applications to signal that the process should exit I am adding a conditional Context interface that applications can implement. When implemented svc will wait for a SIGINT/SIGTERM OR for the context to finish.

[judwhite]

@jehiah Thanks. I'll check and merge over the weekend

[judwhite]

LGTM

[jehiah]

Thanks @judwhite