Add "theme-color" element

[stefanobartoletti]

This is not strictly a SEO property but a more generic meta element, but I think it would be useful to also add support for the theme-color element, which is used in Chrome for Android, it accepts an HEX color value and has this syntax:

<meta name="theme-color" content="#E64545">

Reference: https://htmlhead.dev/#google-android and https://developers.google.com/web/updates/2014/11/Support-for-theme-color-in-Chrome-39-for-Android

[TiagoDanin]

#109