Missing support for `stretch` vendor values

[JasonHK]

• stretch: current spec
• fill: old spec
• -moz-available: Firefox vendor value
• -webkit-fill-available: Chromium vendor value

[AleshaOleg]

Thanks for the report, I'll take a look soon. Don't you want to try to fix it by yourself and send PR?

[JasonHK]

Thanks for the report, I'll take a look soon. Don't you want to try to fix it by yourself and send PR?

@AleshaOleg I'm happy to submit a PR, but I'm not sure how the detection works...

[AleshaOleg]

#204