[Feature Request] Support some html tags

[mkpoli]

I suggest to support some html tags such as (<span>) in order to design more freely.

You can edit it in editing mode, but it should not be displayed in preview mode.

[daed]

I have mixed feelings about this, but I can't think of any substantial reason not to do it.

How should we escape it? That is, if I want to write about <span> tags instead of creating them, what would be preferred? I guess in github you escape them with backslash. Should we do that here as well?