To avoid bumping into the binary size cap every time we add a new extension-specific URL variable, we should move the definition of these variables out of url-replacements-impl.js.

One way to do this is change UrlReplacements service to hold many VariableSource objects instead of the single, kitchen-sink GlobalVariableSource.

class UrlReplacements {
  /**
   * Extensions should use this to add their special URL variables. 
   * @param {!VariableSource} source
   */
  addSource(source);
}

Then, an extension amp-foo can define its own VariableSource and add it to the UrlReplacements service on initialization. UrlReplacements will expand a string by processing all variable sources in the order they were added (this will be unstable but order doesn't matter right now anyways).

Pros: Avoid future bloating of this file. Also looks like we can knock about 0.8KB (1%) off of the v0.js binary size by doing this for existing extension vars.

Cons: We'll lose useful runtime error messages e.g. "can't use variable X because extension Y isn't installed". Also, possible runtime performance difference as we'll end up building/running N small regex instead of a single large regex.

/cc @jridgewell @jpettitt