Add an option to move all JS below the fold

[gigabo]

This can improve the performance of above the fold render by eliminating
resource competition due to network I/O and synchronous JS parse.

This is an opt-in feature triggered by a jsBelowTheFold key in the response
from handleRoute.

handleRoute() {
  return {
    code           : 200,
    jsBelowTheFold : true,
}