validate: add `Diagnostic.Address` (#204)

This change maps `Diagnostic.Address` from `terraform validate -json` to a Go struct field.

validate.go

diff --git a/validate.go b/validate.go
index dbae3ee..4bd4162 100644
--- a/validate.go
+++ b/validate.go
@@ -44,6 +44,7 @@
 type Diagnostic struct {
 	Severity DiagnosticSeverity `json:"severity,omitempty"`
 
+	Address string `json:"address,omitempty"`
 	Summary string `json:"summary,omitempty"`
 	Detail  string `json:"detail,omitempty"`
 	Range   *Range `json:"range,omitempty"`

validate_test.go

diff --git a/validate_test.go b/validate_test.go
index a3e42b6..4c941e6 100644
--- a/validate_test.go
+++ b/validate_test.go
@@ -70,7 +70,12 @@
           "byte": 200
         }
       }
-    }
+    },
+	{
+		"severity": "warning",
+		"summary": "no ice cream",
+		"address": "module.icebox.ice_cream.vanilla"
+	}
   ]
 }`
 	var parsed ValidateOutput
@@ -104,6 +109,11 @@
 					},
 				},
 			},
+			{
+				Severity: "warning",
+				Address:  "module.icebox.ice_cream.vanilla",
+				Summary:  "no ice cream",
+			},
 		},
 	}
 	if diff := cmp.Diff(expected, &parsed); diff != "" {