Utilize URL from rule metadata

The rule URL is now pulled directly from the metadata if it is
available, falling back to the current method of using
`eslint-rule-documentation` if the rule doesn't specify this for itself.

spec/helpers-spec.js

@@ -28,4 +28,21 @@ describe('The helper module', () => {
       expect(Helpers.getFixableRules()).toEqual(['foo'])
     })
   })
+
+  describe('getRuleUrl', () => {
+    it('works with rules that define their own URL', () => {
+      Helpers.updateRules([['foo', { meta: { docs: { url: 'bar' } } }]])
+      expect(Helpers.getRuleUrl('foo')).toBe('bar')
+    })
+
+    it('works with rules defined in eslint-rule-documentation', () => {
+      const url = 'https://github.com/benmosher/eslint-plugin-import/blob/master/docs/rules/no-duplicates.md'
+      expect(Helpers.getRuleUrl('import/no-duplicates')).toBe(url)
+    })
+
+    it('gives a fallback URL on how to add a rule URL', () => {
+      const url = 'https://github.com/jfmengels/eslint-rule-documentation/blob/master/contributing.md'
+      expect(Helpers.getRuleUrl('foo/bar')).toBe(url)
+    })
+  })
 })

src/helpers.js

@@ -222,6 +222,25 @@ const generateInvalidTrace = async ({
   }
 }
 
+/**
+ * Get the URL of the documentation for a rule, either from the rule's own
+ * metadata, from eslint-rule-documentation's known rules, or the fallback URL
+ * on how to add it to eslint-rule-documentation.
+ * @param  {String} ruleId The rule ID to get the documentation URL for
+ * @return {String}        URL of the rule documentation
+ */
+export function getRuleUrl(ruleId) {
+  const props = lintRules.get(ruleId)
+  if (props && props.meta && props.meta.docs && props.meta.docs.url) {
+    // The rule has a documentation URL specified in its metadata
+    return props.meta.docs.url
+  }
+
+  // The rule didn't specify a URL in its metadata, or was not currently known
+  // somehow. Attempt to determine a URL using eslint-rule-documentation.
+  return ruleURI(ruleId).url
+}
+
 /**
  * Given a raw response from ESLint, this processes the messages into a format
  * compatible with the Linter API.
@@ -280,7 +299,7 @@ export async function processESLintMessages(messages, textEditor, showRule, work
     }
 
     if (ruleId) {
-      ret.url = ruleURI(ruleId).url
+      ret.url = getRuleUrl(ruleId)
     }
 
     let range