dashify_headline should return 6 for 7+ level headers

[svisser]

As h6 is the highest header level in HTML, Markdown only supports headers up to level 6. That means dashify_headline('####### aaa ###') (note the 7 dashes) should return 6 instead of 7 (and the same applies for higher headers).

It would be good to hear your opinion as the documentation does not state this explicitly. However, I just checked it in my Markdown site and it does indeed turn the above into a h6 element.

[rasbt]

Another good point. I am not 100% convinced to make a change here though, since I am not converting to HTML here. But I implemented it anyway. I think more people would use 7 pounds by accident then there are people who want to nest one level deeper by adding a not supported 7 pounds headline.
And thanks again :)