Autodetection of custom index page

[carson-katri]

If carton detects an index.html file in the package, it will automatically use that as the custom index page, avoiding having to pass the flag manually.

This could help avoid mistakes where you forget to pass the argument when running dev or bundle. For my app Shhh! I created a simple build script that passes the flag so I don't forget it (among other things) when bundling, but this addition could remove that need altogether.