Remove RouteCollection::addExtensions().

RouteCollection::extensions() can now be used to add/set extensions.
Remove Router::parseExtensions().

Router::extensions() can now be used to add/set extensions.
Better method signatures for Routing.
Correct wording.
Correct doc block.
Update tests.
Update reference to removed method.
Update docblock.
Re-add Router::parseExtensions() for easy migration.

Using it will trigger a warning.
Revert array $options argument to boolean $merge in routing classes.
Merge pull request #4525 from cakephp/3.0-router-extensions

3.0 router extensions