generate ci subcommand

[Gankra]

This can be cut from the milestone but I think it would be Killer if you could do something like:

cargo dist init github-ci

And it just creates .github/ci/cargo-dist.yml or whatever.

However this raises questions about the Scope of cargo-dist's functionality and what the action would cover:

• What would the action react to? (presumably: pushing a tag with vBlah ala)
• Would it create a github release? (https://github.com/taiki-e/create-gh-release-action)
• Would it push packages to a github release (or elsewhere?) (https://github.com/taiki-e/upload-rust-binary-action)

Or is this stuff more in scope for "the axo cli"?

One possible reason to want cargo-dist to be in the loop for ci declaration is for #19. One could image declaring "hey I want these targets built, your figure it out" and dist figures out the ideal matrix of platforms/cross-compilation to do this Best (building apple stuff on non-apple platforms is a pain in the ass that apple doesn't want you doing).

[Gankra]

You can either do cargo dist init --ci=github at first setup or cargo dist generate-ci github to do it later