Conditionally allow assets to be loaded without a manifest. Update README to document available options.

[evanfarina]

This PR is meant to address a part of #87 . In short, we are generating a manifest from our external addon because some of the host apps that consume our addon do not use engines nor do they provide their own manifest through other means. In the event that a consuming host app does use ember-engines we run into an issue where a manifest with the engine name is not generated.

This PR does not address the underlying issue but does solve our particular problem which is that our addon should not need to supply a manifest in the first place. I.e we'd like to re-use the lazy-loading functionality that ember-asset-loader provides without having to deal with manifest generation.

I could use a hand from the community to write a test against this new option. My thinking is that we could write a unit test in asset-manifest-test.js which calls getManifest on the asset-loader service and verifies that no manifest was pushed when the noManifestLookup option is present. For the life of me I cannot remember how to grab a reference to a service from within a unit test using this old qunit module syntax. I'm also coming up empty-handed in the docs. Both this.owner and getOwner(this) both return undefined. I'd really appreciate some help here.