Add OutputAssertionBuilder::empty()

[RazrFalcon]

Currently, the only way to check that there was no output is to use .stdout().is("").

I think that .stdout().empty() will be more elegant.

[epage]

I personally am a little unsure if the case is common enough to warrant its own check when there is a discoverable check that isn't much longer.

FYI because of #70, this will either turn into .stdout(Output::empty) or .stdout(empty).

[epage]

If we move forward with #98 then this will be covered by assert-rs/predicates-rs#25.

[epage]

Addressed in assert_cmd
https://github.com/assert-rs/assert_cmd
assert_cli is going to morph into a wider-focused tool, including things like assert_fs. See #41