Add support for skipping existing assets and entries

[umazalakain]

I created a ticket in contentful/contentful-cli#2638, then realised it was probably better here:

It would be great if there would be an option to skip existing assets and existing content. As far as I understand, the CLI utility correctly identifies what the already existing assets and entries are, however it always updates them. This means there is currently no way of performing incremental space migrations. Updating assets and entries is time consuming (e.g. 32k assets take 3h), so it would be great if we could do this incrementally and only import new assets and entries. This could take the form of --skip-asset-updates and --skip-entry-updates options.