Expose summernote styleWithSpan

Summernote has an option which makes summernote using native HTML tags
instead of using style attributes (see HackerWins/summernote#114).

According to a comment in the summernote code [0], this is only working
in Firefox and Chrome. I haven't checked that.

By default the option is True, which doesn't change default behavior.

[0]
https://github.com/HackerWins/summernote/blob/master/src/js/settings.js