Allow populating strings for module names in builds

[jrburke]

Now that 2.1.2 has a file cache, there are some dynamic build situations, particularly for web-based builders, where the string contents of a file could be auto-generated. Ideally these strings could be seeded into the file cache on each optimizer run.

Consider a build option that looks like this:

{
  rawText: {
    'some/module/id': 'string contents here'
  }
}

Then, in jslib/build.js, once config is set in the requirejs context, but before the first require() for modules happens, go through the rawText list, call require.toUrl to get urls for those IDs, and seed require._cachedRawText with the text values.