Enhancement: add return values to click commands

[mijamo]

Return values in click do not hurt anything nor doe sit have any impact on regular CLI users. However, this would things much easier to work with if somebody wants to build another Click application on top of this.

My use case is that we will need to upload many small tileset with a similar recipe, so I am writing a script that calls this library to do so. I could use the API directly but I would use some of the nice stuff that has already been built in here (ex: upload progress, validation, conversion of files, error handling etc.). Right now I am able to invoke the commands properly (through ctx.invoke) without issue but I cannot really act on their result because they are only sent through echo and not returned.

If you think it is a good idea I can make a pull request to integrate that.