overridable templates to allow html

[PetrDlouhy]

This is my take on #305 - I tried to make the templates overridable in order for user to be able to add |safe easily.

I don't need this solution now, because my menu_bootstrap5.html template is too far from the original anyway. But other users might appreciate this.

I did include some documentation.

@idlesign What do you think of this? Do you want this approach or do you want to choose something different?

[idlesign]

Hi.

Seems ok, yet I wonder how heavy this change is. That is, how does adding blocks affect rendering performance on big menus. Any insights?

Another thought: what if we did it with marking safe in items hook?