new param on probe to get stderr

[ezequielramos]

I need to get the stderr from probe even when it doesn't fail.
The reason why this is necessary is because there're important information to me on "Warning logs" -v warning.

So I created a new parameter capture_stderr with False as default value, what should make older versions working exactly the same way, but when using the True value, the function will return a tuple with the default return value on first position and the stderr on the second position.

[ezequielramos]

If you need me to add a test, let me know.
I have a video to use as example, but it has like 300kb, I don't know if you want me to upload this on the repo.

[ezequielramos]

#614