match block constructs in single pass

Improve on jrblevin#91 by increasing speed of matching block constructs, and
allow them to be nested within each other, by matching all of them in a
single pass.

- Ensure font locking of block constructs is predicated solely on text
properties.
- Add "language" field to tilde fence opening regexp.
- Make `test-markdown-font-lock/pandoc-yaml-metadata` pass.