Support plugins for resolving custom URLs to a list of peers

[prashantv]

Today, -p specifies a single peer, and for HTTP also happens to specify the URL template for the request including the protocol scheme of choice as well as the path. -P specifies a peer list, currently only supporting a file in YAML or JSON or flat text format. We propose to extend -P to accept a URI with multiple supported protocols including file: (to explicate the current behavior), and add http: to support grabbing the content via an HTTP request, and allow registering alternate protocols.

I think the right solution is to use the plugin support in go1.8 to load plugins that can provide custom peer providers.

type PeerProvider interface {
	Schemes() []string
	Resolve(context.Context, *url.URL) ([]string, error)
}

Resolve returns an array of alternative flags for -p, which over HTTP continues to serve as the URL template and can be used in combination with -P. In the absence of a provided -p, the selected peer template string is used both for the address and the template.

$ yab -P alt://blah/blah:http --health

[kriskowal]

Landed in #156