As specified at https://daringfireball.net/projects/markdown/syntax#em
italics are made with either single asterisks/underscores, and bold is
double asterisks/underscores. Before this, single asterisks were
understood as bold, and only underscores were understood as italics;
both of which behaviors are incorrect.
The option is now used as a fallback when detection by extension fails. Some
scripts like `base/mail.kak` and `base/html.kak` still rely heavily on it.
