add xlsx support

by factoring out a Fetcher class

src/lib/Controller.js

@@ -22,6 +22,7 @@ class Controller {
   update () {
     return Promise.all(
       Object.keys(this.fetchers).map(sheet => {
+        console.log(sheet)
         return this.fetchers[sheet].update()
       })
     ).then(results => {