[TwigHooks] Allow discarding hookable's context

[jakubtobiasz]

Currently, automatically we merge the hookable-level context with the hook-level context. So data provided in the entry template is automatically passed down to the most bottom hookable.

In order to prevent this behavior, while declaring a new hook, we are able to add the only keyword, which will disable this mechanism. In such case, only the data from the with keyword will be passed down.

Example:

{% hook 'content' with {
	title: 'some title',
} only %}