Add support for 'missing' as a match key to support ordering on
missing match values, and fix a bug w.r.t. lowercasing match variable
names.
Add 'txt' to code-block declaration, since leaving it off causes the text
to not be rendered.
Escape backslashes so they render as desired.
Merge pull request #737 from mozilla-services/versions/0.5

Add support for 'missing' as a match key to support ordering on