feat: add random background Spoon

[roeeyn]

I've always enjoyed the dynamism of having a fresh background with every Hammerspoon config reload. I have used both the UnsplashRandom and UnsplashZ Spoons in the past, but recently, they've started to exhibit issues. The core of this problem stems from Unsplash's decision to deprecate the source endpoint, which both of these Spoons heavily relied upon.

In light of this, I took the initiative to develop a new Spoon that taps into the modern Unsplash API. The primary distinction with this approach is the necessity of an API key, a measure implemented by Unsplash to safeguard against misuse. However, I am currently in the process of finalizing the best practices for documenting these instructions.

I sincerely appreciate any feedback you might have and look forward to collaborating with the community to make this Spoon the best it can be. 😄

[muescha]

i miss documentation and a variable in obj for unsplash_api_key
like obj.unsplash_api_key = ""

-> since the API call need the key it should be mandatory at start maybe as parameter for start

additionaly I would allow the user to change the interval and the url of the service (in case they change again the api)