Fix the issue of extra </span> after the image

It’s a know issue reported at
https://github.com/robwierzbowski/jekyll-picture-tag/issues/23

It is caused by the unwant <p> tag around the <span> tag.

As the markdown processing will wrap the <span> element with a <p> tag,
so use the {::nomarkdown} {:/} block to skip the markdown processing to
avoid the extra <p> tag.
Merge pull request #25 from raphaelfeng/master

Fix the issue of extra </span> after the image