add "webassets.bundles" configuration that auto-registers bundles from a YAML file

this PR allows the webassets configuration to specify a file or asset that will be loaded via webassets' [YAMLLoader](http://webassets.readthedocs.org/en/latest/loaders.html?highlight=loader#webassets.loaders.YAMLLoader) mechanism. example:

``` ini
webassets.bundles   = %(here)s/path/to/asset.yaml
# or as an asset-spec
webassets.bundles   = package.module:dir/asset.yaml
```

(the asset-spec based approach allows the bundle spec to be fetched from inside a python egg.)