add additional test for reference syntax of images (fixes #75)

lib/rules.js

@@ -1206,7 +1206,7 @@ module.exports = [
   {
     "name": "MD045",
     "desc": "Images should have ALT Text attribute",
-    "tags": [ "images", "accessibility" ],
+    "tags": [ "accessibility", "images" ],
     "aliases": [ "no-alt-text" ],
     "regexp": null,
     "func": function MD045(params, errors) {

test/detailed-results-MD041-MD050.md

@@ -19,3 +19,7 @@ A [normal](link) and an [empty one]() and a [fragment](#one).
 An image without alt text ![](image.jpg)
 
 ![](image.jpg)
+
+A reference image without alt text ![][reference]
+
+[reference]: image.jpg  "title"

test/detailed-results-MD041-MD050.results.json

@@ -45,7 +45,7 @@
         "errorRange": [25, 13]
     },
     {
-        "lineNumber": 22,
+        "lineNumber": 25,
         "ruleName": "MD043",
         "ruleAlias": "required-headers",
         "ruleDescription": "Required header structure",
@@ -97,5 +97,14 @@
         "errorDetail": null,
         "errorContext": null,
         "errorRange": null
+    },
+    {
+        "lineNumber": 23,
+        "ruleName": "MD045",
+        "ruleAlias": "no-alt-text",
+        "ruleDescription": "Images should have ALT Text attribute",
+        "errorDetail": null,
+        "errorContext": null,
+        "errorRange": null
     }
 ]