Enable Safari 10 save mangling (#3)

Safari 10 has an issue with `let` and redeclarations; the only way to safely
let uglify mangle and generate a build capable of supporting Safari 10 is to
enable the specific Safari 10 option.

No.

Seriously.

mishoo/UglifyJS#1753 (comment)

There is no noticable bundle size difference with this enabled

| type | safari10: true | safari10: false|
-----------------------------------------
| uncompressed | 494k | 489k |
| brotli  | 130k | 134k |
| gzip | 154k | 148k |
| deflate | 154k | 148k |

So somehow this is making Brotli more efficient. And the only browser that
doesn't support Brotli? Safari 10.

So win-win everywhere!

Author: toddself

.index.js.swp

@@ -45,6 +45,9 @@ module.exports = function (b, opts) {
   var uglifyOpts = {
     output: {
       ascii_only: true
+    },
+    mangle: {
+      safari10: true
     }
   }
   if (!b._options.debug) {