add pre-defined attrs and link to spec for WebGLContextAttributes
add pre-defined attrs and link to spec for WebGLContextAttributes

magny/DefinitelyTyped,dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,borisyankov/DefinitelyTyped,georgemarshall/DefinitelyTyped,magny/DefinitelyTyped,mcliment/DefinitelyTyped,markogresak/DefinitelyTyped,borisyankov/DefinitelyTyped