Fix keyword expansion and add configuration for auto keyword expansion

[esmarkowski]

This pull request includes fixes for keyword expansion and adds a new configuration option for auto keyword expansion.

• :keyword fields should never need .keyword appended to them
• :text and :string fields get a keyword multifield added by default
• If any field has a multifield it will be used for aggregations, sorting, etc (by adding the name of the path to the :keyword multifield)
• The default keyword multifield is attribute_name.keyword

Configuration

Change the default_keyword_field:

Stretchy.configure do |config|
     config.default_keyword_field  = :whatever
end

attribute_name.whatever

Turn off adding keyword fields to :text (and any subclass of text, like :string):

Stretchy.configure do |config|
     config.add_keyword_field_to_text_attributes  = false
end