JavaScript: incluir arquivos js/css dinamicamente

gistfile1.js

/*
 * Ex:
 * include("http://mysite.com/bookmarklet.css");
 * include("http://mysite.com/bookmarklet.js");
 */
let include = ( src, callback ) => {
    let ext = src.split( /[\#\?]/ )[ 0 ].split( '.' ).pop().toLowerCase();
    let inc;

    if ( ext === 'css' ) {
        inc       = document.createElement( 'link' );
        inc.rel   = 'stylesheet';
        inc.type  = 'text/css';
        inc.media = 'screen';
        inc.href  = src;

        if ( callback ) {
            callback();
        }
    } else {
        inc        = document.createElement( 'script' );
        inc.src    = src;
        inc.async  = true;
        inc.onload = inc.onreadystatechange = callback;
    }

    document.querySelector( 'head' ).appendChild( inc );
};