Detect and respect YAML frontmatter

Github supports [YAML frontmatter](http://krasimir.github.io/techy/docs/#using-yaml-front-matter). One of things about TextMate that has irked me forever is that it treats the initial `---` as a separator and colors it differently. This pull requests establishes the idea of a YAML frontmatter and gives it a context in TextMate. It doesn't do anything with it just yet and themes won't support it, but it will stop TextMate from incorrectly treating the `---` as a line separator in this situation.