Add support for krew indexes

[ajgon]

Krew has built-in external indexes support. It would be nice, if krewfile support that :-). This PR adds a new syntax, using index keyword, while maintaining backwards compatibility.

Example krewfile:

index netshoot https://github.com/nilic/kubectl-netshoot.git

netshoot/netshoot
modify-secret
node-shell
resource-capacity

Also as I bonus I added -dry-run flag, which may be useful for testing.

[brumhard]

@ajgon I really appreciate the effort you put into this. Unfortunately, I'll be busy for the next few weeks, but I'll review it once I find the time 🙏🏻

[brumhard]

@ajgon I'm back. Thanks for you patience. I went over the PR and really like your additions and interface. I will go ahead and merge it, I might refactor a little later, add some docs etc. Thanks for the work you put in 🙏🏻