WIP: Add multi-field support to common term query

Add support for matching multiple fields to the common term query. Like this:
{ "common": {
  "body": {
    "high_freq_operator": "and",
    "fields": ["title", "body"],
    "query": "nelly the elephant not as a cartoon",
  }
} }
The extra fields come from the "fields" array and the name of the object
containing the query denotes the field from which the common-ness is
derived.

WIP needs:
1.  A review on the funky syntax for adding the fields.  Feals bolted on
because it is.
2.  Gotta get the multi match query to use it.
3.  Docs.  Including a working rest example.
4.  More tests!?

Closes #5024