Make the liked icon configurable

[Utagai]

Currently, the icon spt uses to mark liked items, such as a liked song, is the heart character: ♥.

For any variety of reasons, the user may want to change their preferred icon. e.g.:

1. Their font family does not support the glyph.
2. Their terminal does not render it properly.
3. Preference

For me, #1 applies, so I unfortunately have an empty space whenever I see a liked song. I was thinking that we could make the default value stay as the ♥, but allow users to configure a replacement string if they wish.

[Utagai]

@Rigellute Let me know if this is something you'd be OK with in spotify-tui. I would be more than willing to work on this as it affects me currently.

Also, it seems like the way we add on the icon is duplicated throughout the code, making it pretty easily error-prone. Let me know what you think about giving a shot at cleaning that up.

If not, that's cool too. I can work off of my fork.

[Rigellute]

Yep, happy for this to be added to the config 👍

[Utagai]

Great to hear. I'll take a crack at this soon.