Added JSON output of metadata to make parsing independent of tags output

[sashahilton00]

Issue by frafall
Thursday Nov 30, 2017 at 10:30 GMT
Originally opened as plietar/librespot#267

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Just a small step of standardizing metadata by printing it as json, in this implementation I
print artist/title but as output is json we can easily add more tags without breaking stdout parsing in integrations.

ex:
metadata:{"ARTIST":"Pink Floyd","TITLE":"Money"}

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frafall included the following code: https://github.com/plietar/librespot/pull/267/commits

[ComlOnline]

Please see #7 for continued discussion.