Ability to add specific templates for ElementalList children

[christopherdarling]

Similar to https://github.com/dnadesign/silverstripe-elemental/pull/154/files but the way things work right now you can have

BlockName.ss or BlockName_Elements.ss which would target any DataObject with an ElementArea has_one with the name of 'Elements' not so wouldn't necessarily only apply to elements that are children of ElementalList.

The reason for the need for this is that the block component when used on a page has a container markup that isn't needed when the blocks are used as ElementalList children. Probably slightly overlaps with the kind of scenario in silverstripe/silverstripe-elemental#159

Seems like it would be easy enough to implement this via the updateRenderTemplates hook but maybe worth something including in the module?