Adding configuration for endpoints

[kellrott]

Adds --endpoint-config and -l flags to server command line to pass configuration parameters to endpoint plugins.

This changes the signature for an endpoint plug in to:

func NewHTTPHandler(client gripql.Client, config map[string]string) (http.Handler, error) 

[matthewpeterkort]

This PR yields a new non-fatal error for existing grip-endpoints plugins.

[kellrott]

It changes the function signature of the plugin methods. So they need to be upgraded. Example at

grip/endpoints/graphqlv2/handler.go

Line 32 in ac185a0

func NewHTTPHandler(client gripql.Client, config map[string]string) (http.Handler, error) {

[matthewpeterkort]

makes more sense. Thank you.