Support <itunes:order>

[tkem]

From [https://www.apple.com/itunes/podcasts/specs.html#order]:

<itunes:order>

This tag can be used to override the default ordering of episodes on the store.

This tag is used at an <item> level by populating with the number value in which you would like
the episode to appear on the store. For example, if you would like an <item> to appear as the
first episode in the podcast, you would populate the <itunes:order> tag with “1”. If conflicting
order values are present in multiple episodes, the store will use default ordering (pubDate).

[tkem]

Would need real-life examples to see how this is used in practice, especially with regard to browse_order and lookup_order.