Plugin support to add own filetypes

[crunchtime-ali]

As you see the demand for QuickLook plugins is huge and I think it might be worth getting some burder off your shoulders (cc @samuelmeuli) by creating a plugin interface for people to add their own Go or Swift plugins. I think what has been holding back the creation of QuickLook plugins in the past is that it's relatively complicated to someone who never did so.

I am aware that this is no easy feat and that people can also add new file formats via Merge Requests to this repo but what do you think about the idea in general?