[mod] plugin: call on_result after each engine from the ResultContainer

Currently, searx.search.Search calls on_result once the engine results have been merged
(ResultContainer.order_results).

on_result plugins can rewrite the results: once the URL(s) are modified, even they can be merged,
it won't be the case since ResultContainer.order_results has already be called.

This commit call on_result inside for each result of each engines.
In addition the on_result function can return False to remove the result.

Note: the on_result function now run on the engine thread instead of the Flask thread.