Feature Request: Option to use a release tag other than 'latest'

Circup is broken today because there is a bad release, and circup both **requires a download first**, and only downloads the **latest release** from GitHub. There are good reasons to require a download first, and a broken build is rare, but it would be useful to specify a git tag other than the latest release. I think this could be done by passing a tag option to the bundle download function. This tag would default to `latest` but could be overridden from the command line.

Start by locating the bundle download function and the command-line option handling described in the issue. Trace how the download currently selects the latest release, then verify that the tag defaults to `latest` and can be overridden from the command line without changing the existing download requirement.

Written by the indexing model from the issue text.