Prevent feature branch push & track when merge strategy is rebase

This is the implementation from #15 as we discussed.

The fact that you chose a rebase strategy for `zenflow feature update` almost certainly means you do not want to publish your newly created feature branches immediately, so I just made it so, I didn't provide yet another point of friction during `zenflow init` that the user needs to worry about.

However, the thing zenflow does where it pushes and sets the newly created feature branch to track is nice, so I added a new command for 'publish' which is coming in another PR.

I added specs for both, they all pass, and I tested it locally on my current project by bundling to a branch that integrates both of these PRs. I am excite :) Hopefully you guys dig these changes. LMK if you need any tweaks.