Add underline as a formatting option

[vincode-io]

The reason that Zavala doesn’t support underlining styles is that links are also shown using underlines. To support underlining we could change this to have links shown in an alternate color and only show the underline when a link is hovered over.

Motivation: Italic styling doesn't work work the the standard Japanese font Hiragino Gothic and that limits formatting options to just bold for Japanese users.

Related to #198

[vincode-io]

I think the biggest reason why this will probably never get implemented is that there is no underlining in Markdown. Zavala does its best use Markdown for import and exports, so compatibility with Markdown processors is needed. There just isn't a universal extension to the Markdown spec that has underlines.

Some processors that do support underlines in Markdown use conflicting syntax. One example I saw uses tildes to denote underlines. This directly conflicts with the CommonMark Spec which considers that to be a strikethrough.

I'm going to leave this open for now incase anyone else has some solution to this, but it looks like adding underlines causes more problems than if fixes.