Add Support For Custom Objects and Fields

[DanRigby]

https://www.jsonfeed.org/version/1.1/:

Publishers can use custom objects in JSON Feeds. Names must start with an _ character followed by a letter. Custom objects can appear anywhere in a feed.

For instance, imagine a podcast directory service — call it Blue Shed Podcasts — that asks a podcaster to specify some additional information about a feed or item. A publisher would do something like this:

"_blue_shed": {
    "about": "https://blueshed-podcasts.com/json-feed-extension-docs",
    "explicit": false,
    "copyright": "1948 by George Orwell",
    "owner": "Big Brother and the Holding Company",
    "subtitle": "All shouting, all the time. Double. Plus. Good."
}

Feed readers that do not understand a given custom object must ignore it.

The about string is there for a human looking at the feed, so they can understand what goes in the custom extension. It's optional, but it should appear at least once in a feed that may contain multiple uses of _blue_shed (preferably in the first use, but that's not required).

Also, it's good practice to name an extension with a company or service name, to provide a clue right away as to what it's for and who made it.

Further naming rules: the extension name and its member keys must not contain any . characters. The extension name, and only the extension name, must begin with an _ character. (No standard keys will ever begin with an _ — this is reserved for extensions.)

Member keys should be alphanumeric. That said, emojis are not illegal — we're not heartless — and somebody's going to use them.