Added a bare-bones eslint config (removing jslint)

This change removes the small bit of jslint config, replacing it
with eslint. I've currently configured eslint to mostly only report
the more serious of lints, although there are still some style nits
turned on.

Upcoming changes will start fixing lints.

src/librustdoc/html/static/main.js

@@ -10,8 +10,12 @@
  * except according to those terms.
  */
 
-/*jslint browser: true, es5: true */
-/*globals $: true, rootPath: true */
+// From rust:
+/* global ALIASES, currentCrate, rootPath */
+
+// Local js definitions:
+/* global addClass, getCurrentValue, hasClass */
+/* global isHidden onEach, removeClass, updateLocalStorage */
 
 if (!String.prototype.startsWith) {
     String.prototype.startsWith = function(searchString, position) {