add `clientVariable` option to `AngularTemplatesAsset` and update readme

This change is necessary in order to use the `BrowserifyAsset`'s purposed `prepend` option (as per PR #109) with multiple `AngularTemplatesAssets`. Prior to this they always used the `angularTemplates` client side global variable which means that if you "prepend" 2 or more, they overwrite each other and the last one wins.

It can be used stand-alone as well though, which is why I've created a separate PR.