Capturing libsass cerr output for testing purposes

[PolyPik]

Libsass directly writes its deprecation warnings to std::cerr in certain functions.

As a result, it becomes very hard to check these warnings in the tests as there is no error object being returned once the sass module has finished rendering (no exception thrown means no error object). Existing packages and methods for capturing the stderr output only works for output coming from the Javascript code and not output coming from native modules.

[saper]

Should this go into libsass repo?