Make h1 titles default and change CLI switch

As I commented elsewhere:

> [This PR](https://github.com/gollum/gollum/pull/889) fixed a bug that broke the --h1-titles switch; that is, gollum was always using h1-titles, whether or not the switch was passed or not. Now that we fixed it h1-titles is again disabled by default. I'm wondering whether it makes sense to change the default to h1-titles, and provide a --no-h1-titles switch. I personally think h1 titles look better, and it looks like everyone has been using them unwittingly because of this bug. Without --h1-titles you get things like this:

@sunny @bartkamphorst what's your opinion on this? Please merge or close. :)