Move to generic context variable instead of $Rule

[BernieWhite]

Originally the intent of the $Rule variable was to provide context about the current rule.

Over time $Rule has had additional properties added to it. Additional context properties and potentially helper methods will be required (i.e. #321). Based on this it makes sense to return $Rule to a pure rule context instead of a generic pipeline context.

An additional $PSRule context variable will be created to handle generic context information.

Existing TargetName, TargetType and TargetObject properties on $Rule should be deprecated in favor of $PSRule.