A cleaner and more readable postprocessors.content-format

[musjj]

The format template for the metadata postprocessor requires you to insert \n for newlines because of JSON's limitations.
But this gets unreadable real quick if your templates are complex. I'm not sure what the best solution would be, but how about using a list of strings that gallery-dl can '\n'.join(list) later on?

So instead of doing this:

"postprocessors": [
    {
        "name": "metadata", "mode": "custom", "extension": "tags",
        "content-format": "tag1:{tag1}\ntag2:{tag2}\ntag3:{tag3}\ntag4:{tag4}\ntag5:{tag5}\ntag6:{tag6}\ntag7:{tag7}"
    }

]

You do this:

"postprocessors": [
    {
        "name": "metadata", "mode": "custom", "extension": "tags",
        "content-format": ["tag1:{tag1}",
                           "tag2:{tag2}",
                           "tag3:{tag3}",
                           "tag4:{tag4}",
                           "tag5:{tag5}",
                           "tag6:{tag6}",
                           "tag7:{tag7}"]
    }
]