CLI flag --detached on `cypress open`

[brian-mann]

Instead of cypress open always opening a detached process we should change that to not open a detached process but accept a new flag --detached which will replicate the existing behavior.

[RandallKent]

The code for this is done, but this has yet to be released. We'll update the issue and reference the changelog when it's released.

[brian-mann]

Fixed in 0.20.0