Feature: Counter cache

[endel]

Allow belongs_to relationships to cache the number of related rows.

Like Rails: http://apidock.com/rails/ActiveRecord/Associations/ClassMethods/belongs_to

[endel]

YAML relationship configs will looks like this:

authors:
  relationships:
    has_many:
      - posts
    counter_cache:
      - posts

posts:
  relationships:
    belongs_to: authors

[endel]

As we're discussing on #130, the counter cache feature may be defined like this:

authors:
  has_many: 
    - posts:
        counter_cache: true # creates a field with name "books_count"