[Snackbar] Support for second action / click to close

[toddmedema]

Problem: Snackbars for persistent / important things should be harder to accidentally dismiss, but should still be dismissible by the user.

Example: Specifically, looking at the Google Inbox design, they have a "Your version is out of date" snackbar (which is the use case we're emulating) that does not auto-hide nor disappear if the user clicks anywhere on the page. Rather, it has a second action on the right side of the snackbar with a "X" (that's the same color as the text - the main call to action is still a different color). Clicking the X closes the snackbar.

We've looked at rolling our own snackbar to achieve this, but it really seems like something that'd benefit + make sense for the larger material-ui community.