WIP - Adding isCSSVisible assertion

[scalvert]

First pass at resolving #418.

I'm open to having a different approach.

TODO:

• Generate docs
• Add optional options object with count (TBD)

[scalvert]

@Turbo87 interested in your thoughts here. If we don't feel this has legs I'd rather just close it.

[Turbo87]

@Turbo87 interested in your thoughts here

TBH I think the best way forward might be to remove the isVisible() assertion completely. There are just too many ways that this could lead to false positives or negatives and I think adding a second isVisible() assertion variant will likely make the situation worse.