Add "Show in Folder" to context menu

Based on @watson's PR #463.

Differences:

- Remove the "Open Folder" link from expanded torrent view.
- Use showItemInFolder instead of openItem electron API
- Add a separator
- Use IPC to invoke electron.shell.showItemInFolder from main process

main/ipc.js

@@ -70,10 +70,15 @@ function init () {
   })
 
   ipcMain.on('openItem', function (e, path) {
-    log('opening file or folder: ' + path)
+    log('open item: ' + path)
     electron.shell.openItem(path)
   })
 
+  ipcMain.on('showItemInFolder', function (e, path) {
+    log('show item in folder: ' + path)
+    electron.shell.showItemInFolder(path)
+  })
+
   ipcMain.on('blockPowerSave', blockPowerSave)
 
   ipcMain.on('unblockPowerSave', unblockPowerSave)