Add roles param for GET /wp/v2/users #386

I can get all users for a single role fairly easily, via the filter query parameter:

/wp-json/users?filter[role]=contributor

However, filter[role] doesn't seem to support the comma syntax that I can use in other commands:

// Doesn't work:
/wp-json/users?filter[role]=contributor,author
// I'd have expected it to work like this:
/wp-json/posts?filter[author]=17,4,164

Is there any way through the API to query for users of multiple roles? At present I'm having to do this by chaining batches of queries, and even with promises that request syntax is nasty. I'd much prefer to get them as a single collection.

Related #924