gtest should support output stream redirect

The default reporter (PrettyUnitTestResultPrinter) writes straight to stdout via printf calls. I would like to have an API so I can provide my own std::ostream, like boost Test library: https://www.boost.org/doc/libs/1_68_0/libs/test/doc/html/boost_test/test_output/logging_api/log_ct_output_stream_redirection.html

This is useful when you use gtest as part of a bigger framework and you need to redirect to output somewhere else (or convert it on the fly to a different encoding for example).

If for running the tests I can at least work around the problem by writing an event listener and writing into my own stream, for *listing* the tests, there is no workaround since `ListTestsMatchingFilter` is not accessible and the listeners are not invoked for the list tests action.