Add possibility to choose to use underscore for italics

[bugsysop]

"You can use asterisks and underscores interchangeably for emphasis markup. Using one for italics and one for bold is the best way to clearly distinguish intention within the text. I personally like underscores for italics and double asterisks for bold, but I don’t care when reading someone else’s text, as long as it’s consistent." Write better Markdown --- Brett Terpstra http://brettterpstra.com/2015/08/24/write-better-markdown/