chore(npm): add "textlintrule" as keywords

textlint/textlint#253

Author: azu

package.json

@@ -27,7 +27,8 @@
     "test": "mocha"
   },
   "keywords": [
-    "textlint"
+    "textlint",
+    "textlintrule"
   ],
   "description": "ら抜き言葉を検出するtextlint rule",
   "devDependencies": {
@@ -45,4 +46,4 @@
     "kuromojin": "^1.2.1",
     "textlint-rule-helper": "^1.1.4"
   }
-}
+}