Ensure that the html() helper works

[rich-iannone]

Similar to #13, we need to make the html() work with tab_header() (and other functions where new labels are accepted, eventually). Example:

import gt
from gt import exibble, countrypops, gtcars, md, html

from siuba import *

res = gtcars >> select(_.mfr, _.model, _.msrp) >> _.head(5)

gt.GT(res).tab_header(
    title=html("Data listing from <strong>gtcars</strong>"),
    subtitle=html("From <span style='color:red;'>gtcars</span>"),
)

[rich-iannone]

This is now implemented in a recent commit. And it’s now required if you want to use HTML strings since input text is now HTML-escaped.