Makes Zencoder path configurable.

This makes it possible to configure the destination path of files
processed by zencoder in the pageflow initializer.
Merge the path as the key `:path` into `zencoder_options`, for example:

    config.zencoder_options.merge!(
        [other options]
        :path => "/:zencoder_asset_version/custom/part/:id_partition/:filename"
    )

Paperclip interpolations apply.