Add support for ignoring properties in Compass::PropertyWithMixin

You can now keep Compass properties from being checked by adding them to
an 'ignore' list in your config file:

  Compass::PropertyWithMixin:
    enabled: true
    ignore:
      - opacity
      - inline-block

Change-Id: I22625f461f5e49b969de47644c136619dd3c53e6
Reviewed-on: http://gerrit.causes.com/40955
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Add spec for new PropertyWithMixin ignore option

Change-Id: Ifc7d552844e8bdc9c117c2719b53ef9f76d6bb40
Reviewed-on: http://gerrit.causes.com/40956
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Add docs for PropertyWithMixin ignore option

Change-Id: I38b3334a6cf2210f76cfbdd1dcb4c95e4d3842e7
Reviewed-on: http://gerrit.causes.com/40957
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>