Honor filtering of Stylesheet URI to allow child themes.

[kopepasah]

Currently, there is no way to use a child theme with AMP due to the hard coded get_template_directory() . '/style.css'.

https://github.com/Automattic/amp-wp/blob/89e4227edf41a8fe0408e8e855a6dcbbd4ccc13c/includes/class-amp-theme-support.php#L391

As I can see it, there are two options:

1. Use get_stylesheet_uri(): this would allow child themes to work and allow users to customize the location of the stylesheet (we usually keep it stored in an assets/css directory).
2. Use get_stylesheet_directory() . '/style.css': this is still a bit restrictive, but would allow the style.css file to either be from the child or current theme. Since child themes require a style.css file, this should always be present when using a child theme.

[westonruter]

@kopepasah Please give #887 a try. It tries to make enqueueing CSS much more seamless. The only requirement is that you enqueue CSS that exists on the filesystem in a theme/plugin, wp-admin, or wp-includes. It also handles added inline styles, locale stylesheets, and Custom CSS.