Allow indefinite duration for showing errors

[alexhelder]

The Material Design Spec Sections "App Errors" and "Incompatible state errors" show snackbars being used to communicate different error states:
http://www.google.com/design/spec/patterns/errors.html#errors-app-errors

"Consider displaying an unintrusive but persistent indicator when users are in these states."

However the current implementation requires a definite duration to show. I propose adding a constant SnackbarDuration.INDEFINITE which would keep it on screen, needing to be dismissed by swipe or action.

[wmora]

sounds good, will work on it when I get a chance

[arturdryomov]

👍

[dirkam]

+1

[wmora]

@alexhelder @dirkam @ming13 released in v2.7.0

[arturdryomov]

Thanks! 🎄