Reimplement first heading becomes header

[mikael-ros]

But this time, leave header functionality in, and make it so not just # heading becomes a header, but specifically # **heading**. This avoids preventing certain layouts, but also lets the raw markdown stay readable.

There should not be any reason why anyone would bold a heading anyway, is the idea, since afaik the heading is bold by default, which is why it would be a decent compromise for those seeking portability and readability.