[target] allow for resolvers other than net.Resolver #130
Description
in kubernetes, in our environment, we can't ping a service ip.
i have the need to ping the endpoints backed by a service (the pods themselves) so i have patched ping_exporter to use a Resolver interface which defaults to net.Resolver as the code currently does in master, but implements a k8s client based resolution of endpoints objects from a service's name and allows users to configure a resolver: label to set the resolver to k8s.
would this be a welcome PR?