Allow - in disk snapshot names

It appears that - was omitted from the possible character in snapshot
names.
It doesn't seem like there is any issue when using - in the name, as
I was able to create snapshots with dashes both from the portal and
through this plugin (with this patch applied).

azurerm/resource_arm_snapshot.go

@@ -216,10 +216,10 @@ func resourceArmSnapshotDelete(d *schema.ResourceData, meta interface{}) error {
 }
 
 func validateSnapshotName(v interface{}, k string) (ws []string, errors []error) {
-	// a-z, A-Z, 0-9 and _. The max name length is 80
+	// a-z, A-Z, 0-9, _ and -. The max name length is 80
 	value := v.(string)
 
-	r, _ := regexp.Compile("^[A-Za-z0-9_]+$")
+	r, _ := regexp.Compile("^[A-Za-z0-9_-]+$")
 	if !r.MatchString(value) {
 		errors = append(errors, fmt.Errorf("Snapshot Names can only contain alphanumeric characters and underscores."))
 	}