Add keywords to improve searchability

My first searches for this component were "react range slider" and
"react range input". So adding these keywords should help this component
to show up in search results.

Also, the closest HTML 5 input to this is `<input type="range">`

See: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/range

package.json

@@ -6,7 +6,9 @@
     "react",
     "react-component",
     "react-slider",
-    "slider"
+    "slider",
+    "input",
+    "range"
   ],
   "homepage": "http://github.com/react-component/gesture/",
   "repository": {