Allow single kw in :order-by and other clauses

[p-himik]

Fixes #467

Also fixes not being able to use helpers to merge into stuff like {:select :id} - e.g. (hh/select {:select :id} :x) would throw.

I would also update README so it mentions the "plain" syntax earlier and uses it wherever possible.

[seancorfield]

I'll think about the documentation aspect. This was a new "feature" in v2.x that fell out of an implementation detail for the most part. I'll create an issue for doc updates.