Remove k3sup app command in favour of arkade

Changelog for 0.9.0:

• PR #214 Add tests for thrown errors upon project-id omission by @utsavanand2

In this version we removed the "k3sup app" command in favour of "arkade", you can find out more via #217 and if you're not ready to upgrade, just pin the version to an older release.

We now have:

k3up install - build a new k3s server
k3sup join - build an agent and join it to a k3s cluster

Commits
59a8fa9 Merge branch 'remove-apps' by @alexellis
25e4872 Remove apps from k3sup README by @alexellis
6184746 Remove apps and move to arkade by @alexellis
971f87d Add tests for thrown errors upon project-id omission by @utsavanand2

Generated by Derek