feat: a `stage` command which stages hunks to a specific plan automatically

[rorybyrne]

It would be nice if git-plan could automatically associate each hunk with an existing plan, so that coders don't need to stage their changes into commits manually.

[rorybyrne]

I think the easiest way to achieve this, as a first attempt, is to parse annotated comments in code. For example:

# git-plan: GP-131
def some_func():
    call_stuff()
    ...

When you run git plan stage --plan GP-131, we should parse the files and stage the above hunk because it reference the plan with id GP-131.