Support pagination when fetching workflows.

[chrisgavin]

We currently only fetch a maximum of 100 workflows from the repository if it's not checked out locally. Once cli/go-gh#23 is implemented, we should enable pagination so we can fetch all of them.

Alternatively we could use a different library for accessing the GitHub API such as go-github, but this then requires getting the authorization token out of the CLI config, which go-gh doesn't currently seem to offer a method for.