Add a `next` (and maybe `previous`) command.

[JDPDO]

Hey again 😃

The problem

Sometimes I want to skip a song fast. The loading screen (when the app gets the current status from spotify) in this scenario adds overhead time.

The solution

You do not have to wait for the loaded status to skip a song if you could type sp next (and sp previous) immediately.

@the-lay If it suits you I would prepare a fork. 😉

[the-lay]

Hey hey!

Thanks for the suggestion!
No worries, I will implement it myself today, it should only be two new queries in on_keyword_query and an updated help menu :)

[the-lay]

Try out the latest version and let me know what you think!
I decided to also replace most icons with the actual spotify icons, I think this looks much neater now.