Allow rules to register against multiple ast nodes

Update the AddRule interface to allow rules to register interest in
multiple ast.Nodes. Adds more flexibility to how rules can work, and was
needed to fix the hard coded credentials test specifically.

rules/errors.go

@@ -50,14 +50,12 @@ func (r *NoErrorCheck) Match(n ast.Node, c *gas.Context) (gi *gas.Issue, err err
 	return nil, nil
 }
 
-func NewNoErrorCheck(conf map[string]interface{}) (r gas.Rule, n ast.Node) {
-	r = &NoErrorCheck{
+func NewNoErrorCheck(conf map[string]interface{}) (gas.Rule, []ast.Node) {
+	return &NoErrorCheck{
 		MetaData: gas.MetaData{
 			Severity:   gas.Low,
 			Confidence: gas.High,
 			What:       "Errors unhandled.",
 		},
-	}
-	n = (*ast.AssignStmt)(nil)
-	return
+	}, []ast.Node{(*ast.AssignStmt)(nil)}
 }