Add volume slider to player toggles

[eralpsahin]

Implemented the feature request from #147

Additions

• Adds a volume slider with similar style to main slider below Playback speed input under player-toggles menu in index.pug
• Similar to playback speed previous settings persist between podcasts.
• Finally, handles mprisPlayer similar to other handlers but, I am not sure if that is necessary. I don't understand why it is being checked.

TODO

• Adds translation for en.json. Any help is appreciated for others. If it is okay, I can translate and add to other locales as well.
• Also added TODO for adding keyboard shortcut handlers for volume change. I can add those as well.

If you want me to add anything, I am happy to.

[z-------------]

First of all, kudos and big thanks for looking through my poorly organized codebase and adding in this feature.

I've added in the keyboard shortcuts and a couple of other things.

The mpris stuff is for Linux users who want to be able to control CPod using their environment's interfaces. For example,

if (cbus.audio.mprisPlayer) {
  cbus.audio.mprisPlayer.volume = cbus.audio.element.volume;
}

simply tells mpris that we have changed the volume so that it will be reflected in the mpris interface.