Support for new sw-toolbox options

[jeffposnick]

The underlying sw-toolbox library recently added support for a networkFirstTimeout option and should be adding support for a maxCacheEntries option soon.

Once those are both available, we should modify <platinum-sw-cache> to expose properties that configure those options.

[cherukumilli]

@jeffposnick - anyone working on this issue? If not, can you please assign this issue to one of the contributors? I would love to have the networkFirstTimeout option.