Add support for "keyfiles" to sublime-build syntax highlighting

[keith-hall]

while looking at https://forum.sublimetext.com/t/trying-to-create-a-debian-package-sublime-build/34430, I noticed that keyfiles wasn't highlighted correctly. This PR corrects that and adds the relevant completions. More info at:

• http://www.sublimetext.com/docs/3/build_systems.html#options
• https://forum.sublimetext.com/t/custom-build-system-for-rails/24609/2
• https://forum.sublimetext.com/t/build-systems/14435
• https://forum.sublimetext.com/t/dev-build-3072/14605

[keith-hall]

This now also fixes a bug whereby the cancel key in build systems was updated in recent dev builds, but the highlighting was scoping valid constructs as invalid and didn't recover gracefully either.