Add option to configure custom theme hook suggestion

In D7, menu_block provided theme hook suggestions that were based on the page region so that themers could alter the HTML based on page position.

In D8, blocks don't have access to the region or any page-positioning context. They are meant to be rendered and cached with only their config and placed independently.

Since we can't do page-position-based theme suggestions in D8, so we should provide a direct way for site builders to specify a custom theme hook suggestion.