html5 writer: wrap image elements in <p> unless inline or in figure.
In HTML, <img> is by default an inline element.
The rST "image" directive generates block-level elements
(unless used in a "substitution" directive).
Wrapping in a container enables more advanced CSS styling.
A paragraph as wrapper fits well with Docutils wrapping
inline content in paragraphs by default.
No change for the "html4css1" writer (ensure backwards compatibility).
git-svn-id: http://svn.code.sf.net/p/docutils/code/trunk@8510 929543f6-e4f2-0310-98a6-ba3bd3dd1d04