Add config option for ZeroUnit to exempt certain properties

[cvrebert]

There are some cases where it's necessary to include a unit for 0 values of certain properties, for the sake of compatibility with buggy browsers (e.g. flex for IE10-11 per https://github.com/philipwalton/flexbugs/blob/master/README.md#4-flex-shorthand-declarations-with-unitless-flex-basis-values-are-ignored ). It would therefore be nice if the ZeroUnit linter had a config option which was a list of properties to exempt from the unitless-zeros requirement.