Add back support for the assetPlugin option

Summary:
**Summary**
Metro used to have support for "asset plugins", which allowed developers to specify arbitrary JS modules that could export a function for adding more fields to asset data objects. Some of this functionality was removed in the delta bundler work -- this PR adds it back.

**Test plan**
Made existing unit tests pass and added unit tests to test asset plugin behavior. Also tested E2E in a React Native project by adding `assetPlugin=/path/to/pluginModule` to a JS bundle URL and ensuring that the plugin ran.
Closes https://github.com/facebook/metro/pull/118

Differential Revision: D6711094

Pulled By: rafeca

fbshipit-source-id: f42c54cfd11bac5103194f85083084eef25fa3cd

jest/preprocessor.js

@@ -52,6 +52,7 @@ module.exports = {
       filename: file,
       localPath: file,
       options: {
+        assetDataPlugins: [],
         dev: true,
         inlineRequires: true,
         minify: false,