isNotVisible fails when element is hidden using visibility:hidden style

[BillyRayPreachersSon]

Presently, assert.dom(el).isNotVisible() will fail if the element in question has been hidden using the visibility: hidden style.

Is this intentional?

If not, and it's just an oversight, how do people feel about me raising a PR to update the isNotVisible method to take this style into account (possibly with a flag in a new options parameter, to avoid breaking existing functionality)?

I might also include a .doesNotHaveStyle assertion in the same PR, as right now I can test whether the style has been applied with .hasStyle({ visibility: 'hidden' }, but not for the opposite case.

[steveszc]

Related to #418
I did a little digging and found out why this is the case. TLDR isVisible and isNotVisible are based on jquery's :visible pseudo selector.
#418 (comment)