syntax: support embedding one language grammar in another.

Proof of concept, which provides an excellent environment for
editing Specl spec-files - yaml structure with Lua examples.
* lib/zile/bundle.lua (compile_pattern): Done strip include
string here.
* lib/zile/syntax.lua (leftmost_match): Add repo parameter.
(parse): Adjust accordingly.
(leftmost_match): If include names another grammar, switch to
that grammar's repo and patterns before recursing, otherwise
strip leading '#' and look up in the current repo as before.
* lib/zile/grammar/yaml-bundle.lua (lua): Adjust to use the
`source.lua` grammar for highlighting embedded lua examples.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>