Syntax highlighting for :markdown filter is too greedy

[TrevorBurnham]

Here's a test case:

!!!5
  p
    :markdown
      This looks fine.
    p Why is this blue?

This renders as

http://cl.ly/030K0s3J1D1g3l0n1v2v

Jade correctly interprets the p as being a separate element, not part of the Markdown, so the highlighting is inaccurate.