googletest

Clarify the polymorphic matcher concept and explain how to define a composite matcher.

Allow `DistanceFrom()` to use user-defined `abs()` by default.

Add a `DistanceFrom()` matcher for general distance comparison.

Add a `testing::ConvertGenerator` overload that accepts a converting functor. This allows the use of classes that do not have a converting ctor to the desired type.

Try to warn the user when test filters do not match any tests