syntax: add support for basic syntax highlighting.

* lib/zile/grammar/lua-bundle.lua: New syntax grammar for Lua
language.
* lib/zile/syntax.lua: New file. Return a sparse array of
attributes for a line of text according to that buffer's grammar.
* build-aux/config.ld.in (file): Add lib/zile/syntax.lua.
* local.mk (dist_modules_DATA): Add doc/modules/zile.syntax.html.
* lib/zile/bundle.lua: Use lrex onigurama engine for easier
porting of TextMate grammar files.
(load_grammar): Initial massaging of a grammar file for fast
access to attributes.
* lib/zile/zile.mk (dist_pkgdata_DATA): Add lib/zile/syntax.lua
and lib/zile/grammar/lua-bundle.lua.
* lib/zile/buffer.lua (buffer_new): Initialise syntax member.
* lib/zile/file.lua (find_file): Load the grammar when opening a
new file.
* lib/zz/main.lua (main): Highlight the scratch buffer correctly.
* lib/zile/term_redisplay.lua (draw_line): Use it to set
attributes as the line is displayed.
* lib/zile/lib.lua (stack): Stack object place holder.
* rockspec.conf (dependencies): Add lrexlib-oniguruma.
* travis.yml.in (install): Install libonig-dev package for Travis
CI.
* .travis.yml: Regenerate.
* NEWS: Update.

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