Add a command for the fix command

[isottellina]

This adds a function to call cargo fix in the current project.

I needed a binding to use cargo fix in my projects, and I thought it could potentially benefit other people. Don't hesitate to close the PR if this is not welcome or this matter was already settled and I missed it.

I took the decision to allow staged files to be fixed by default, as that was more useful for me, but I'm open to discussion, of course. It could also be done by the prefix argument (e.g. 4 is, as always, customize the cargo command, but 16 could be allow-dirty and 64 don't allow anything, or in another order)