Added support for required property

[FezVrasta]

Fixes #136.

[ljharb]

LGTM.

Note that the UX around HTML5 form validation can't be customized, so it's rarely advisable to enable it - but adding this is straightforward 👍

[coveralls]

Coverage remained the same at 88.043% when pulling ca6dc25 on FezVrasta:master into 5db44d2 on airbnb:master.

[FezVrasta]

I think it's the exact opposite, I use it exactly because it's a standard across the whole browser, so the people don't have to learn a new pattern on each website.

Also, you can intercept the HTML5 validation event and replace it with whatever you want if you prefer.