Block level referencing and reuse syntax

[lcnbr]

Lots of PKM software use markdown and each implement their own version of embedding parts of one page/document into another. Since we can already attach attributes to blocks, it would be great to have a syntax for referencing/ inputting or embedding the content.

[mcookly]

Perhaps if #199 is implemented, PKM software can read and render those inclusions as an embedded reference.