Title: Blogs API: categories for Wix (and WordPress)

Hi Zernio team,

We're starting to use the new Blogs API to publish to our customers' Wix sites.
We publish two kinds of content there: regular blog posts and knowledge-base
articles. Wix gives each site a single blog, so both end up mixed together in
one feed. We'd like to keep the knowledge-base articles in their own Wix Blog
category, so they get their own overview page on the site.

Right now the Blogs API has no way to set a category. Could you add:

1. A `categories` field on create and update article (POST/PATCH
 /v1/accounts/{accountId}/blogs/{blogId}/articles), taking category names.
 It would work the same way `tags` already does: existing names are
 matched case-insensitively and missing categories are created on the site.
2. `categories` in the article response (get and list), so we can read back
 what was assigned.
3. Optionally, GET /v1/accounts/{accountId}/blogs/{blogId}/categories to list
 the categories that already exist on the site.

As far as we can tell, Wix's own Blog API already supports this through
`categoryIds` on draft posts. WordPress has categories too, so the same field
there would keep the API consistent across platforms. Wix is our priority,
though.

Thanks! You've delivered on our earlier requests really quickly, and that's much appreciated.