Add offset and number to create-guest-authors

[aaronjorbin]

Currently, create-guest-authors is an all or nothing proposition. What happens though when you are 50% complete with a 30,000 user import and then accidently close your terminal window? You have to start over. By allowing chunking of create-guest-authors, this can be continued from the number that have been completed rather than starting from scratch.

Anecdotally, running it in chunks seems to be quicker.

[GaryJones]

Only the last commit is relevant here. Simple but useful addition.