feat: add recursive option, defaults to true

[ehowey]

Hi John et al.,

My use case is that I have a fairly high number of starters all within one directory in a monorepo. I don't want a single change to one starter to trigger the publish workflow across all starters. So I modified this to add a "recursive" option. This probably isn't a perfect or final API, but more an idea. This is also my first time writing bash scripting, so I wouldn't merge this. Moreso it is a proof of concept.

The recursive option is set to default to true, so that means the action continues to function as-is for current users. However when set to false the folder specified is exact and no recursive searching is completed. For example if the folder is specified as "starters/gatsby-starter-awesome" then it will only look in that one specific folder and not recursively inside of it.

I hope that makes sense, I would be happy to explain or discuss more if needed.