Merge branch 'support-xml-json-yaml-for-metadata' into 'master'

Merge branch 'support-xml-json-yaml-for-metadata' into 'master'

Support XML, JSON, and YAML for metadata

Add support for app metadata files in JSON, XML, and YAML data formats.  All of the formats use the exact same metadata tags, so there is no translation layer needed.  They all just parse the data into the same internal data format: Python dicts.  Supporting these standard formats will make it much easier for people to write recipes since they can choose a data format that they are familiar with.  It also makes it much easier to generate metadata programmatically, since there are good libraries for working with all three formats in basically every language (unlike FDroid's .txt format).

Here are the same tags in .txt, JSON, XML, and YAML:

    Source Code:https://github.com/SMSSecure/SMSSecure
    "Source Code": "https://github.com/SMSSecure/SMSSecure",
    <string name="Source Code">https://github.com/SMSSecure/SMSSecure</string>
    Source Code: https://github.com/SMSSecure/SMSSecure

Looking for comments, suggestions, flames, etc. from @CiaranG, @mvdan, and everyone else.

See merge request !57