Allow mixing of multiple blocks/elements on one tag

[Roman Komarov]

Proposed syntax:

+b.mediaList
  +b(['entity', '__item', 'media__'])

Note that the leading element delimiter marks the element and the ending one is marking the context for the nested entities. If there is no context marked, the first block would get it.

[Roman Komarov]

Was done using the usual syntax:

+b.mediaList
  +b.entity.__item.media__