Adds ability to override [flex] attributes under different media queries...

[jamesspencer]

... as described in #706

[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for the commit author(s). If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.

[jamesspencer]

Ack, used wrong email when submitting the PR. I've changed my GitHub public email to the same as that which I signed the CLA with. LMK if you would like this submitted again!

[alanrussian]

I believe this would only account for the 100% case (e.g., flex-phone="100"). It would not account for something like flex-phone="25" which worked before (as flex-sm).

[jamesspencer]

@alanrussian the 100% case appears to be missing from the latest release, AFAICT the intermediary cases (flex-phone="25") are still working. Or do have I you confused?

[ajoslin]

Good call! merged.