Add opt-in Discord-style __underline__ rendering

[PiotrCzapla]

New underline option (off by default). When enabled, Discord-style __x__ renders as <u>x</u> instead of <strong>x</strong>:

to_xhtml("__important__", underline=True)   # <p><u>important</u></p>

[jph00]

I'm open to this, although my preference would be to use ++ instead, since that's a known markdown-it extension and doesn't require a new option. Other possibilities that work now are [x]{.underline} (Pandoc's HTML writer special-cases that class and emits <u>) or just Raw <u>x</u>

WDYT?

[PiotrCzapla]

I played with this yesterday using our offer letter template, and I would leave it as it is, the __ does not add much value.

The .underline class was a better fit for places where underlining was used in the offer letter. It was used as a mini-header:

The proposal looks okay:

__Confidential Information__. The Employee understands and acknowledges that, 

But we would likely want to add an ID to that text, which makes this markup a bit too exotic:

__Confidential Information__{#cinfo}. The Employee understands and acknowledges that, 

Using Pandoc's []{.underline} feels better:

[Confidential Information.]{#cinfo .underline} The Employee understands and acknowledges that, 

Or, we could use headers and simply add the style in the DOCX file:

#####  Confidential Information. {#cinfo}
The Employee understands and acknowledges that, 

Regarding Markdown-it, I wasn't aware that ++ means underline; it looks more like <ins>.

I would close this unless you feel that it would be a good addition.

[jph00]

Heh you're right, ++ means insertion, but it's displayed as underline. :) I'll close this since I don't feel strongly either way, but if you ever decide you do want this after all, I'd need minimal convincing to add it.