CustomAnalyzer ctor to set AnalyzerBase.Type

It is not obvious that the Type property should be set on a class that inherits from CustomAnalyzer.

By providing a ctor that sets the Type property users will not need to remember
to set the type property manually.

Removing the default ctor is ideal, but would be a breaking change.  Marking it obsolete
should lead most people to "the pit of success".