Automatically pause syncing for specific folders while on metered connections

Feature request #39 highlights a very important enhancement to the app, but I believe that there is a more specific configuration that could be of even greater use.

Since not all folders’ content might have the same weight (e.g. a folder could contain Markdown files only, another one videos only) it would be super useful to give users the possibility of choosing **what folders to sync or not** while connected to a metered connection.