Custom SW filename (#175)

* embed: support using custom filename for service worker instead of sw.js
can be set via <replay-web-page swName='custom.js' ... > property
docs: update embedding docs to mention new option
fixes #160

embed.html

@@ -9,7 +9,7 @@ body {
 </head>
 <body>
 
-<replay-web-page replaybase="./" embed="replay-with-info" src="./docs/assets/tweet-example.wacz" url="page:0"></replay-web-page>
+<replay-web-page swname="custom.js" replaybase="./" embed="replay-with-info" src="./docs/assets/tweet-example.wacz" url="page:0"></replay-web-page>
 
 </body>
 </html>