Pandas Series should provide to_html method
Hello,

Pandas Series provide `to_json` method but they doesn't provide `to_html` method.
Adding `to_html` will avoid user to convert a `pandas.core.series.Series` to a `pandas.core.frame.DataFrame` in order to output HTML file using `s.to_frame().to_html(...)` but directly use `s.to_html(...)`

see also https://github.com/pydata/pandas/issues/8825

Kind regards
