Add prepend information to fastboot-dist/package.json

[danmcclain]

Grabs the prepend option set when fingerprinting the files and appends it to the package.json file so that ember-fastboot-server can remove the prepend details. Used by the changes added here

This PR plus ember-fastboot/fastboot#11 closes #102

For asset-rev to manage the fastboot-dist/package.json, 'json' must be added to the replaceExtensions setting in brocolli-asset-rev

[danmcclain]

@tomdale thoughts? I've verified that the combination of these 2 PRs allows for prod build

[tomdale]

After discussing this with @danmcclain, I had concerns about requiring manifest consumers to apply a set of arbitrary operations on the manifest to get the correct list of files—at minimum, it seems like the operations should be applied on the generation side, not the consumption side. Consuming the manifest should be very straightforward and not require additional code.

I am going to take a stab at updating the Ember CLI addon to take into account the broccoli-asset-rev rewriting, by not generating the manifest until it has applied its renaming trees.