perf: 60ms improvement: Replace fs-extra with mkdirp/rimraf/cp-file

In Electron apps, the cost of large modules is very real.

fs-extra is very convenient, but removing it caused 50 fewer unique
files to be required(), resultin in 60ms faster startup!

Before: 557 unique requires (1330-1340ms)
After: 507 unique requires (1270-1280ms)

src/renderer/controllers/subtitles-controller.js

@@ -1,5 +1,5 @@
 const electron = require('electron')
-const fs = require('fs-extra')
+const fs = require('fs')
 const path = require('path')
 const parallel = require('run-parallel')