CSS: Add support to TEI:ab

[lucaju]

As per @jamescummings request:

tei.css should include the follow rule

/** JC adding tei:ab basic formatting **/

ab  {
    display: block;
    margin-bottom: 0.5em;
    margin-left: 1em;
	line-height: 1.5em;
}

James explanation:

tei:ab element is 'anonymous block'. It is basically a paragraph or blob of text without the semantics of a paragraph. The canonical example (no pun intended) is verses of the bible in English. They are not verses so not (metrical lines), but they are also not paragraphs in themselves. So verse of bible text here.