`content_text` incorrectly takes precedence over `content_html` when parsing JSON Feed#492

`content_text` incorrectly takes precedence over `content_html` when parsing JSON Feed, making it impossible to get `content_html` if both exist. https://github.com/kurtmckee/feedparser/blob/e43242143ed90ee8cbf05078faf972f8de35a798/feedparser/parsers/json.py#L88-L97 According to https://www.jsonfeed.org/version/1.1/, `content_text` and `content_html` are completely equal. > `content_html` and `content_text` are each optional strings — but **one or both** must be present. > Note that it uses **both** `content_text` and `content_html`, which is **completely valid**. An app such as iTunes, for instance, might prefer to use `content_text`, while **a feed reader might prefer `content_html`**. Thus, a better methodology to parse it may be adopting the Atom approach: making `entries[i].content` a dict array, i.e., `[{"type": "text/plain", "value": "content"}, {"type": "text/html", "value": "<p>content</p>"}]`. Such a change, admittedly, would break existing downstream projects using the `develop` branch. Hopefully, this won't be painful, considering JSON Feed support hasn't been released yet. I am willing to make a PR to achieve this if you think this is feasible.