Add support to aggregate multiple queries into a single response

[php-coder]

It should be possible to return a multiple members where each of them are came from different queries.

• python support
• golang support
• javascript support
• add documentation
• add tests

[php-coder]

Example of a response:

{
  "num_of_categories": 5,
  "num_of_countries": 10
  "num_of_series": 100
}

And possible syntax:

- path: /v1/stat
  get:
    queries:
      num_of_categories: SELECT COUNT(*) FROM categories
      num_of_countries:  SELECT COUNT(*) FROM countries
      num_of_series:     SELECT COUNT(*) FROM series

OR

- path: /v1/stat
  get:
    aggregated_query:
      num_of_categories: SELECT COUNT(*) FROM categories
      num_of_countries:  SELECT COUNT(*) FROM countries
      num_of_series:     SELECT COUNT(*) FROM series

OR

- path: /v1/stat
  get:
    query:
      aggregate:
        num_of_categories: SELECT COUNT(*) FROM categories
        num_of_countries:  SELECT COUNT(*) FROM countries
        num_of_series:     SELECT COUNT(*) FROM series