[REQ] Relax enclosing block parsing

[mhatzl]

Enclosing blocks should still be considered valid
if there is no blank line after the end sequence.
This reduces possible expensive backtracking during parsing.
Especially in case attributes are set for a block, because block and attribute parsing would need to be revoked if not followed by a blank line.

Note: Compilers should issue a warning, if enclosed blocks are not followed by a blank line.

Valid with this change:

[[[
Valid block, but warning
]]]{
  id: some_block;
  color: grey;
  font-family: serif;
  font-size: 16px;
}
No blank line after end of enclosed block.
Will lead to a warning, but is still correctly parsed.