Add playlists feature (#871)

* Open multi-file torrents as playlists

* Add `repeat` and `shuffle` options to the player

* Autoplay first file in torrent

* replaces `pickFileToPlay` feature
* when reopening player, restores the most recently viewed file

* Add playlist navigation buttons to Windows thumbar

* Remove `repeat` and `shuffle` options

This reverts commit 9284122461.

* Play files in order they appear in torrent

* Clean up playlists code

src/renderer/lib/errors.js

@@ -1,8 +1,15 @@
 module.exports = {
-  UnplayableError
+  UnplayableTorrentError,
+  UnplayableFileError
 }
 
-function UnplayableError () {
+function UnplayableTorrentError () {
   this.message = 'Can\'t play any files in torrent'
 }
-UnplayableError.prototype = Error
+
+function UnplayableFileError () {
+  this.message = 'Can\'t play that file'
+}
+
+UnplayableTorrentError.prototype = Error
+UnplayableFileError.prototype = Error