Perf: lazy-load more require() calls in main process

Went from 36 unique require calls, to 31 calls after this commit.

src/config.js

@@ -1,5 +1,4 @@
 const appConfig = require('application-config')('WebTorrent')
-const fs = require('fs')
 const path = require('path')
 const electron = require('electron')
 const arch = require('arch')
@@ -146,6 +145,8 @@ function isPortable () {
     return false
   }
 
+  const fs = require('fs')
+
   try {
     // This line throws if the "Portable Settings" folder does not exist, and does
     // nothing otherwise.