add support for src attribute in strapdown.js

load-xml-src.js

<!-- usage -->
<!-- <xmp src="homepage.md" theme="cerulean" style="display:none;"></xmp> -->
<!-- <script src="load-xmp-src.js"></script> -->

var elements = document.body.getElementsByTagName("*");
for(var i=0; i<elements.length; i++) {
    var element = elements[i];
    if(element.nodeName == "XMP") {
        var xhr = new XMLHttpRequest();
        xhr.open('GET', element.getAttribute("src"), false);
        xhr.send();
        element.innerHTML = xhr.responseText;
    }
}

Thanks. Perfect. 👍