Implements `index: { defined: true }` index

[zedtux]

This PR adds the defined: true option to the index method allowing to only index documents having the given field.

class MyDoc
  include NoBrainer::Document

  # Using the field options
  field :field1, index: { defined: true }
  field :field2
  field :field3

  # Using the index method
  index :field2, defined: true
end

The following queries will use the above declared indexes:

MyDoc.where(:field1.defined => true).count

MyDoc.where(:field1.defined => true, :field2.defined => true).count

[zedtux]

This doesn't work on RethinkDB side without a clear understanding of the issue (discussed on Slack) therefore I'm closing it.