Enable dry-run for pre

[jpnelson]

It'd be nice for our release to know in advance which packages will be released.

Pre knows this, but it also affects files on the file system.

It'd be nice to have a dry-run option that would disable writing the results, or maybe to split out pre into a separate step that would tell the next step what packages will get released. This next step could do the IO. Might be related to #52