Highlight Assert::get_output in docs

[epage]

From @passcod on #74

It's not obvious that to get the output of a command without doing asserts on it (e.g. if I want to use another assertion tool) I have to still go through the Assert trait. I ended up writing a helper to read stdout and stderr to string via the Command handle but later saw I could have just used get_output().stdout.to_string()...

I'm thinking we should have a note in the assert documentation mentioning the ability to access this for completely custom logic outside of the scope of assert_cmd