Add the itunes channel "itunes:type" tag.

[mindreader]

This adds support for the itunes channel tag itunes:type which is meant to mark the podcast as either serial or episodic in nature. Episodic podcasts are like news podcasts where you only want the most recent episodes, serials are podcasts that are meant to be listened from the start, like the appropriately named podcast "serial".

https://help.apple.com/itc/podcasts_connect/#/itcb54353390

[codecov[bot]]

Codecov Report

Merging #101 into master will increase coverage by 0.02%. The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #101      +/-   ##
==========================================
+ Coverage   79.52%   79.54%   +0.02%     
==========================================
  Files          24       24              
  Lines        1724     1731       +7     
==========================================
+ Hits         1371     1377       +6     
- Misses        353      354       +1     

Impacted Files	Coverage Δ
src/extension/itunes/itunes_channel_extension.rs	82.71% <83.33%> (+0.04%)	:arrow_up:
tests/read.rs	95.31% <100.00%> (+0.01%)	:arrow_up:

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[andy128k]

Code is OK. But I am not sure that podcast_type is a good name. It could be r#type to reflect a tag's name or show_type to be more generic and conform to Apple's description.

[mindreader]

I didn't know you could use a keyword that way. This works best for me as I'm using it from elixir and having it come through as "type" makes the most sense.