syntax: select the appropriate mode according to file extension

* lib/zile/grammar/diff-bundle.lua: New file, with the syntax
grammar for diff and patch files.
* lib/zile/zile.mk (dist_pkgdata_DATA): Add diff-bundle.lua.
* lib/zz/default-color-bundle.lua: Set highlight styles for new
diff grammar.
* lib/zile/bundle.lua (load_file_associations): Load a map of
file extension to grammar basename using the fileTypes key in
each grammar file.
* lib/zile/term_redisplay.lua (draw_status_line): Display the
name of the loaded grammar for the current buffer in its mode-
line.
* NEWS: Update.

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