Implement flag "resolvers". (#24)

* Propagate errors.
* Use junit test output.
* Expand role of DecodeContext to include Scanner.
* Inject resolved flags as Path elements in the Context.
  This allows all existing logic to apply seamlessly: hooks, required
flags, etc.
* Clarify that hooks can be called multiple times.

help_test.go

@@ -8,6 +8,7 @@ import (
 )
 
 func TestHelp(t *testing.T) {
+	// nolint: govet
 	var cli struct {
 		String   string `help:"A string flag."`
 		Bool     bool   `help:"A bool flag with very long help that wraps a lot and is verbose and is really verbose."`