startup perf: Reduce require() calls

Every require() that we do before the users sees UI reduces startup
time.

I used the following code (added to index.js) to log every require()
call in the main process:

```js
var Module = require('module')
var required = {}
Module.prototype.require = function (orig) {
  return function (id) {
    if (!required[id]) {
      required[id] = true
      console.log(`${id}   (from ${this.filename})`)
    }
    return orig.apply(this, arguments)
  }
}(Module.prototype.require)
```

From this I was able to learn that lots of modules were being required
that aren't actually used until later.

I also sent this related PR to eliminate another few require()s:
https://github.com/LinusU/node-application-config/pull/4

This increases startup time by 50ms.

We'll probably realize much bigger gains by following this same
procedure for the renderer process.

src/main/menu.js

@@ -13,8 +13,6 @@ const electron = require('electron')
 const app = electron.app
 
 const config = require('../config')
-const dialog = require('./dialog')
-const shell = require('./shell')
 const windows = require('./windows')
 
 let menu = null
@@ -90,17 +88,26 @@ function getMenuTemplate () {
             ? 'Create New Torrent...'
             : 'Create New Torrent from Folder...',
           accelerator: 'CmdOrCtrl+N',
-          click: () => dialog.openSeedDirectory()
+          click: () => {
+            const dialog = require('./dialog')
+            dialog.openSeedDirectory()
+          }
         },
         {
           label: 'Open Torrent File...',
           accelerator: 'CmdOrCtrl+O',
-          click: () => dialog.openTorrentFile()
+          click: () => {
+            const dialog = require('./dialog')
+            dialog.openTorrentFile()
+          }
         },
         {
           label: 'Open Torrent Address...',
           accelerator: 'CmdOrCtrl+U',
-          click: () => dialog.openTorrentAddress()
+          click: () => {
+            const dialog = require('./dialog')
+            dialog.openTorrentAddress()
+          }
         },
         {
           type: 'separator'
@@ -277,18 +284,27 @@ function getMenuTemplate () {
       submenu: [
         {
           label: 'Learn more about ' + config.APP_NAME,
-          click: () => shell.openExternal(config.HOME_PAGE_URL)
+          click: () => {
+            const shell = require('./shell')
+            shell.openExternal(config.HOME_PAGE_URL)
+          }
         },
         {
           label: 'Contribute on GitHub',
-          click: () => shell.openExternal(config.GITHUB_URL)
+          click: () => {
+            const shell = require('./shell')
+            shell.openExternal(config.GITHUB_URL)
+          }
         },
         {
           type: 'separator'
         },
         {
           label: 'Report an Issue...',
-          click: () => shell.openExternal(config.GITHUB_URL_ISSUES)
+          click: () => {
+            const shell = require('./shell')
+            shell.openExternal(config.GITHUB_URL_ISSUES)
+          }
         }
       ]
     }
@@ -361,7 +377,10 @@ function getMenuTemplate () {
     // File menu (Windows, Linux)
     template[0].submenu.unshift({
       label: 'Create New Torrent from File...',
-      click: () => dialog.openSeedFile()
+      click: () => {
+        const dialog = require('./dialog')
+        dialog.openSeedFile()
+      }
     })
 
     // Edit menu (Windows, Linux)