<i>italics</i> in citation text

Citation text does not rely on text styling … **except** for a need for _italics_ (e.g., for publication titles).  This need is not specifically addressed in the GEDCOM X specification—a `SourceCitation.value` is currently a plain text string.

To address this need, my thought is that we constrain the `value` field as follows:

>REQUIRED.  This string is plain text, except when a text region needs to be italicized. If a proper citation requires a region of text to be italicized (e.g., a book title), surround the region with an xhtml `i` element.  An application rendering a citation `value` SHOULD render a region(s) surrounded by an xhtml `i` element in an _italic style_ without rendering the `i` element markup. If an application implements only a plain text rendering of the citation `value`, the `i` element markup SHOULD be included in the rendering.

The proposed modification is provided here in a pull request.