Add Polymer to list of FormatJS integrations

[stramel]

Polymer has a way to integrate formatjs into Polymer applications. Might be good to provide this information in the list of integrations?

https://elements.polymer-project.org/elements/app-localize-behavior

https://www.polymer-project.org/1.0/toolbox/localize

[ericf]

Yeah, sure. Feel free to send a PR if you have time.

[stramel]

@ericf Sorry for the late PR.

[stramel]

Resolved in #216