Adds support for `publicPath` to enable serving assets from different locations. (#299)

Summary:
<!-- Thanks for submitting a pull request! Please provide enough information so that others can review your pull request. The two fields below are mandatory. -->

**Summary**

<!-- Explain the **motivation** for making this change. What existing problem does the pull request solve? -->

This is an updated followup from https://github.com/facebook/metro/pull/280, which we would still need to address the following assumptions about `/assets/`:

e7deea1900/packages/metro/src/Server.js (L332)

e7deea1900/packages/metro/src/Server.js (L379)

e7deea1900/packages/metro/src/Server.js (L385)

As pointed out by gdborton, there isn't currently a way to make this a configurable option. I am not certain how to find a proper workaround for `processRequest`.

We found a temporary solution to have our express app pick up serving the assets from a publicPath as a middleware. But the change in this PR is still necessary to get it working fully.

**Test plan**

<!-- Demonstrate the code is solid. Example: The exact commands you ran and their output, screenshots / videos if the pull request changes UI. -->

- Will add tests once we figure out a comprehensive solution
Pull Request resolved: https://github.com/facebook/metro/pull/299

Reviewed By: rafeca

Differential Revision: D12939229

Pulled By: mjesun

fbshipit-source-id: 769c23468c5ac434f8319e5e7caaf46dd6453f2d

jest/preprocessor.js

@@ -59,6 +59,7 @@ module.exports = {
         minify: false,
         platform: '',
         projectRoot: '',
+        publicPath: '/assets',
         retainLines: true,
         sourceType: 'unambiguous', // b7 required. detects module vs script mode
       },