Add keybindings to Pithos

Additions as listed below:

| Keystroke | Action             |
------------|--------------------|
| `Return`  | Play selected song |
| `C-l`     | Love current song  |
| `C-b`     | Ban current song   |
| `C-→`     | Skips to next song |
| `C-↑`     | Increase volume    |
| `C-↓`     | Decrease volume    |