[mod] json_engine: add content_html_to_text and title_html_to_text

Some JSON API returns HTML in either in the HTML or the content.
This commit adds two new parameters to the json_engine:
content_html_to_text and title_html_to_text, False by default.

If True, then the searx.utils.html_to_text removes the HTML tags.

Update crossref, openairedatasets and openairepublications engines