floating point matchers for std::complex<T>

[quantumsteve]

Description
Describe the feature/change you request and why do you want it.

Our application needs to compare complex numbers for equality. We created a ComplexApprox class to provide this functionality.

std::complex<double> i(0.,1.), j(0.,1.);
CHECK(i == ComplexApprox(j));

We want to make an upstream contribution. Since Approx is deprecated could a custom matcher provide the same functionality? Is such a pull request likely to be accepted?