Add zIndex property

[maackle]

This is the only property I've found that takes a dimensionless value (Int in this case), so I just show it to String, rather than creating a newtype and corresponding Val Int instance.

Closes #65

[CLAassistant]

All committers have signed the CLA.

[maackle]

FYI the CI build failed because of something unrelated:

$ npm run -s build
* Error: Unrecognised arguments: --censor-lib, --strict
Usage: pulp [global-options] <command> [command-options]

[garyb]

Thanks! Int makes sense here for sure.