Consider Markdown header as post title

Post titles are currently considered (for text posts) as the first 17 characters of the post. When a Markdown header is set at the top of the post, I propose to choose its content as post title. In all other cases the old behavior is chosen.

Example of post :

> \# Header \#
> is considered as title

Title value is `# Header #\r\nis co...` with the old behavior but `Header` with this PR.

Headers of the following form (setext-style) are also considered :
> Header
>  =====

or

> Header
>  \-\-\-\-\-\-\-\-\-\-