Add embark --step

[kevin]

Would it be useful to apply revisions incrementally without needing to look up and use their ids?

Eg. instead of...

$ jrny embark --through 5
$ jrny embark --through 6

... would it be helpful to have...?

$ jrny embark --step
$ jrny embark --step

Note: This would need to be mutually exclusive with --through. See clap-rs/clap#2621