Add support for "emphasis" markdown shortcuts

[bacoords]

What problem does this address?

Right now Gutenberg has mixed support for markdown-style text shortcuts. You can use shortcuts to generate heading blocks (#), lists (-), and the horizontal rule (---). Using the ` (backtick) character will create an inline code tag for you. But there are a few others that could be helpful while writing.

What is your proposed solution?

Similar to the way Gutenberg recognizes the backtick character and converts it to code, I would like to work on extending that support to emphasis as the scope of this issue.

The ideal result here would be that users could type a single or double asterisk * or underscore _ to get <em> or <strong> tags inline.

https://daringfireball.net/projects/markdown/syntax#em

[Mamaduka]

If you want to give it a try, check out the __unstableInputRule property for Code format.

Swapping the const BACKTICK = '`'; with const ASTERISK = '*'; should probably be enough for essential support.

[bacoords]

Thanks for the pointer @Mamaduka ! - You're right that emphasis was easy enough. I am seeing that the harder part is that in markdown, a *single asterisk* translates to <em> but a **double asterisk** translates to <strong> so I think I need to put a little more logic for that.

[Mamaduka]

IIRC formats can only support a single tagName; based on that "limitation", your PR shouldn't be enough for the core.

A plugin can register a custom format that handles "strong importance" for a text.

[bacoords]

I've updated the PR. There should only be a single tagName added- <em> or <strong>. There needed to be some additional logic in italic/index.js to make sure that it worked correctly and I'll admit that I left the logic a bit more verbose so it could be more clear what was happening. Open to any suggestions on tightening it.