Replace %sapper.main% with %sapper.scripts%

[Rich-Harris]

In Sapper < 0.4, the client-side app is inserted with this line in template/2xx.html:

<script src='%sapper.main%'></script>

We need to change that so that Sapper has more control over what is inserted, so that (for example) we can add a JSON blob of data generated in preload on the server, to fix #3.

As of version 0.4, including %sapper.main% will cause an error on startup, linking back to this issue. Instead, templates should include `%sapper.scripts%:

-<script src='%sapper.main%'></script>
+%sapper.scripts%

[Rich-Harris]

Closed by #87