Support ATTR_HTML for paragraphs and other Org elements

[kaushalmodi]

Then it should be possible to do something like:

Some text not highlighted

#+ATTR_HTML: :class highlight
Highlighted text using CSS with rules set for `.highlight`

Again non-highlighted text. 

---

• paragraph
• example-block
• src-block
• plain-list
• quote-block

---

Not doing anything special for latex environment as org-html-latex-environment will handle it.

[kaushalmodi]

• Support for ATTR_HTML above quote blocks
• Support for ATTR_HTML above plain lists