Add a cursor to go to the first page

[GCorbel]

I added first_cursor to Mongoid::Scroll::Scrollable::Iterator.

first_cursor = nil

cursor = cursor_type.from_record Feed::Item.last, field_name: field_name, field_type: field_type

Feed::Item.asc(field_name).limit(2).scroll(cursor) do |_, iterator|
  first_cursor = iterator.first_cursor
end

Feed::Item.asc(field_name).limit(2).scroll(first_cursor) do |record|
  # loop over the 2 first records.
end

This is honestly kind of the same than Feed::Item.asc(field_name).limit(2) but allows to keep sort options encrypted the same way it is for other kind of cursors.

We use it to have this kind of payload for an API response :

GET {{baseUrl}}/attachments?pageToken=eyJ2YWx1ZSI6bnVsbCwiZmllbGRfdHlwZSI6IlRpbWUiLCJmaWVsZF9uYW1lIjoiY3JlYXRlZF9hdCIsImRpcmVjdGlvbiI6MSwiaW5jbHVkZV9jdXJyZW50IjpmYWxzZSwidGllYnJlYWtfaWQiOm51bGwsInR5cGUiOiJuZXh0In0=
{
    "records": [
        // ....
    ],
    "paging": {
        "pageToken": "eyJ2YWx1ZSI6MTM0OTM2MTI4MS4wMjcsImZpZWxkX3R5cGUiOiJUaW1lIiwiZmllbGRfbmFtZSI6ImNyZWF0ZWRfYXQiLCJkaXJlY3Rpb24iOjEsImluY2x1ZGVfY3VycmVudCI6dHJ1ZSwidGllYnJlYWtfaWQiOiI1MDZkOWU4MTdhYTU4ZDEyNTkwMDBmMTIiLCJ0eXBlIjoibmV4dCJ9",
        "perPage": 1,
        "firstPageToken": "eyJ2YWx1ZSI6bnVsbCwiZmllbGRfdHlwZSI6IlRpbWUiLCJmaWVsZF9uYW1lIjoiY3JlYXRlZF9hdCIsImRpcmVjdGlvbiI6MSwiaW5jbHVkZV9jdXJyZW50IjpmYWxzZSwidGllYnJlYWtfaWQiOm51bGwsInR5cGUiOiJuZXh0In0=",
        "nextPageToken": "eyJ2YWx1ZSI6MTM0OTM2MTI4MS4wMjcsImZpZWxkX3R5cGUiOiJUaW1lIiwiZmllbGRfbmFtZSI6ImNyZWF0ZWRfYXQiLCJkaXJlY3Rpb24iOjEsImluY2x1ZGVfY3VycmVudCI6ZmFsc2UsInRpZWJyZWFrX2lkIjoiNTA2ZDllODE3YWE1OGQxMjU5MDAwZjEyIiwidHlwZSI6Im5leHQifQ=="
    }
}

Given that, we can go to the first page at any time.

[dblock]

Looks good. Add to README please.

[dblock]

Suggested change

	* [#42](https://github.com/mongoid/mongoid-scroll/pull/42): Add a cursor to go to the first page - [@GCorbel](https://github.com/GCorbel).
	* [#42](https://github.com/mongoid/mongoid-scroll/pull/42): Add a `first` cursor - [@GCorbel](https://github.com/GCorbel).

I think for #38 above we should say "Add a previous cursor" too?

[GCorbel]

I changed.