Rename `title` to `header`

[epage]

This is inspired by #114 where someone wanted to include a lot of details in the "title" but assumed they couldn't.

[epage]

If we decided to do this, we can do it without a breaking change with the current API because we can add a new function and deprecate the old one.

[epage]

Alternatively, from #114

It's much more intuitive for me when constructor methods are named after the thing they construct (unless they construct Self, in which case some variant of "new" or "from"). So I'd personally name that method "message" and call its parameter "header".

[epage]

We're looking at

• Title -> Message
• Group::with_title stays the same

We went with Message because

• Title has problems mentioned
• Header feels weird to go after a Snippet to describe it
• rustc calls it this