Add <link rel="preload"> for needed scripts when exporting

[Rich-Harris]

Sapper adds preload headers when the server is running, but that doesn't work with sapper export

[Rich-Harris]

fixed by #568

[khrome83]

@Rich-Harris - this appears to be broken now. I am not see this work in the most recent version.

[maxmilton]

@khrome83, thanks for your feedback. How about openning a new issue and adding some info about the regression? Otherwise your comment may go unnoticed.

[khrome83]

@maxmilton - I can do that. No real steps to reproduce though. It just does not exist...