More This Like Query: Added syntax for single item specification.

Users are more likely to search for only one document than for multiple ones.
This commit extends the current syntax with 'id' and 'doc' for single item
search. Additionally, 'ids' and 'docs' now support a single value or a single
object respectively. For consitency with 'like_text', alternative syntax such
as 'like_doc', 'like_docs', 'like_id' and 'like_ids' are also possible.