Fixes #3: --only-collection exports only first collection

I have fixed #3, but I do however think the whole current approach of the script is off. Currently it is trying to make a new collection for every single resource group, which seems a bit OTT. I work on multiple APIs and each one of those is its own collection, and not just each resource for a specific API.

With this PR, the `--only-collection` switch will merge all resource groups into one collection. This seems logical in my use-case, but I am struggling to work out what is the best fit for this script without changing the whole darn thing.