Add setting for text formatting type in Markdown

[WhenLifeHandsYouLemons]

There should be a setting that asks whether doing Ctrl+I and Ctrl+B should add formatting in the editor and not the preview (as it currently does), or whether it should add * for italic and ** for bold, or whether it should add _ for italic and ** for bold.

[WhenLifeHandsYouLemons]

The choice between _ or * shouldn't be there as * is more versatile and better to use than the _.