Rename `--prepare-for` to `--edition`, drop arg

Rename `--prepare-for` to `--edition`, drop arg

This commit tweaks the UI of `cargo fix` for the edition. Previously you'd
execute `cargo fix --prepare-for 2018`, but that's a lot of typing! Plus it's
some manual data that Cargo can already infer.

Instead, after this commit, you now type `cargo fix --edition`, and that's it!
The idea is that this'll tell Cargo to fix code for the *next* edition,
inferring whatever edition is in use and figuring out what to pass to rustc.

Functionality-wise this should be the exact same as `--prepare-for 2018` though

If others agree w/ this change I'll send a PR to the edition guide after this
merges!