Add possibility to choose to use ATX Headers

[bugsysop]

"Use ATX Headers (i.e. hashmarks). Setext headers (dashes or equal signs immediately after a headline) are harder to visually differentiate from other syntax when scanning, plus they only go up to 2 [levels], so you’re using ATX headers after that anyway. Be consistent." Write better Markdown --- Brett Terpstra http://brettterpstra.com/2015/08/24/write-better-markdown/