An isolated subcontext inherits the environment, filters,
and static registers of its supercontext, but with a fresh
(isolated) scope.
This will pave the way for adding the `render` tag, which renders
templates in such a subcontext.
Users of Liquid will often wish to filter an array to only those items that match a certain criteria. For example, showing "pinned" messages at the top of a list.
Example usage:
`{{ comments | where: "pinned" | first }}`
or
`{{ products | where: "category", "kitchen" }}`
* Add where filter to standard filters
* Add tests for new where functionality
Samuel