add template caching

[marconi]

When using tooltip-template-url, don't refetch template if there's a cache for it.

[45kb]

@marconi Hi, have been thinking about this feature, your work is great just this has to be an option something like tooltip-template-url-cache="true" so that i can choose if to use it or not.

Obviously every cached template must have an index so it will not cache every template for every tooltip element, but only the ones i would, but i think this is obvious 👍