Add `getTtl` function to WrapConfig options

[jpwilliams]

This function could receive the data ouput from the work function given and return a ttl number for the psetex in _pushData.

This would cover many cases, but still not the HTTP example in the event where the resulting data is the document itself but the TTL comes from the headers.

[jpwilliams]

Whatever route, WrapConfig should definitely include a ttl option so we can override the global value per call.

To solve the issue in the HTTP example, though, we could add a boolean WrapConfig option which denotes that the return value of the passed function is a new interface which returns a ttl and data.

interface WrapResult {
	data: any;
	ttl?: number;
}

This feels shit, though, as it requires people to repurpose functions.

Maybe adding both this and a getTtl function in WrapConfig would be good? That way the user could choose the less invasive option.