CSS tweak to documentation.
Undefined variables are now rendered as blank strings (not verbatim as
they were previously). This change is because unescaped undefined
variables would be silently rendered it a variable with the same name
were defined sometime in the future.

Added variable reference `{variable-name?default-value}` syntax to allow
undefined variables to render a default value.

Added `--prepend` option to rimuc command.
Bump version number.