Add subquery count

[fabricereix]

Add an optional subquery to a query.

Currently, only capture could have a subquery, it is better to generalize to the query so that it can be used in both captures and asserts.

Define the subquery count which will be applied to a collection returned by the main query.
The count result can then be used with existing predicates such as equals or greaterThan.
The existing predicates such as countEquals will be therefore deprecated.

jsonpath "$.books" count == 10
jsonpath "$.books" count > 10

[fabricereix]

implemented in #219