Raise exception on bad list items

[crdoconnor]

Issue raised by @Carpetsmoker in #23:

foo:
  - one
  - two
    - three

is (yaml module):

{'foo': ['one', 'two - three']}

or (strictyaml module):

YAML(OrderedDict([('foo', ['one', 'two - three'])]))

Desired outcome: some kind of exception. Maybe prevent multiline list items unless | is used.

Input and comments on this issue welcomed.