Adding item itunes:episodeType tag.

[mindreader]

One last tag. <itunes:episodeType>, which can be either "episodic" or "serial". If it is "serial", then apple mandates that you must have episode numbers on your episodes.

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

[codecov[bot]]

Codecov Report

Merging #103 into master will increase coverage by 0.03%. The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master     #103      +/-   ##
==========================================
+ Coverage   79.58%   79.62%   +0.03%     
==========================================
  Files          24       24              
  Lines        1739     1747       +8     
==========================================
+ Hits         1384     1391       +7     
- Misses        355      356       +1     

Impacted Files	Coverage Δ
src/extension/itunes/itunes_item_extension.rs	85.18% <83.33%> (-0.15%)	:arrow_down:
tests/read.rs	95.36% <100.00%> (+0.02%)	:arrow_up:

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[AnderEnder]

@mindreader, if there are only 3 types, why String is used, I would expect Enum with 3 values.