Add extra_properties option to PropertySpelling

There are a lot of CSS properties, and the lack of a definitive list (as
well as the constant introduction of new experimental properties) makes
it impractical to ensure our list is up-to-date at any given time.

To address this, add an `extra_properties` config option to the
`PropertySpelling` linter so that developers can add additional
properties to the whitelist without needing to submit a pull request.

Change-Id: I60f250cb704f2245e66885caada8929c37485076
Reviewed-on: https://gerrit.causes.com/32423
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>