Auto names for elements based on blocks

[kizu]

Sometimes we could add names for different elements based on parent block names. It would be especially helpful for lists, so (with addition of #23):

+b.list
  +e foo
  +e bar

would be rendered as

+b('ul').list
  +e('li').item foo
  +e('li').item bar

(basically — if the element is a list item and is unnamed — name it as item etc.)