Add Google Podcast extension

[OpenHaus]

Google integrated Podcast support into the play store last year. [1]

Accompanying they also published a few tags to optimize the podcast listing. [2]

It would be nice to see this supported by Zend Feed as an extension.

[1] https://support.google.com/googleplaymusic/answer/6343833?hl=en
[2] https://developers.google.com/search/docs/data-types/podcast

[froschdesign]

The tags are similar to iTunes. Example:

<googleplay:block>
    yes
</googleplay:block>
          OR
<itunes:block>
    yes
</itunes:block>

https://developers.google.com/search/docs/data-types/podcast#podcast-level-rss-tags

[weierophinney]

Thanks, @chrisdeeming.

[michalbundyra]

@froschdesign why this is reopened?

[froschdesign]

@webimpress
This issue doesn't belongs to #68. (@weierophinney used the wrong number in the commit message)