Add raw_html parameter to Styler.format, enabling richer output
`Styler.format` could allow a parameter `raw_html` (bool, False for current behaviour), enabling richer output

For example, `df.style.format(lambda s: f"<a href='https://en.wikipedia.org/wiki/{html.escape(s)}'>{html.escape(s)}</a>", raw_html=True)` could be used to turn a column of values into a column of links to English Wikipedia articles, when exported with `to_html`.

To achieve this, we would need to always pass raw HTML to the jina templates, changing how formatting is currently done in `Styler.to_html`.

Formats performed with `raw_html=True` would have no effect on `Styler.to_excel`.