Import relationships (Categories, Entries, Assets) via ID as an option.

[radarseven]

First off, this plugin is fantastic! Thanks for all of your hard work on it. This is an enhancement request.

I've been working on importing a set of data that contains multiple relationships (Category) across the datasets. I know the protocol for importing relationships, Categories specifically - the Categories need to have URL's and the title in the import data need to match the title of the related Category.

This has worked fine for me in the past. However, what I'm experiencing this time around is that any Category title with a special character in it, for example a slash /, is not found on import.

As an alternative, which would solve this issue, would be to have the option to lookup the related Category by ID instead of title and slug. This would ensure better results on importing those relationships.

[damianDauntless]

Please see my pull request, I was able to achieve Entries Relationship via ID. #164