Allow slide numbering in revealjs

[kcrisman]

In the place where the revealjs conversion initializes itself and things like the navigation mode, it would be pretty easy to allow slide numbers as well, e.g. after this spot one could add

<xsl:text>  slideNumber: true,&#xa;</xsl:text>

Most likely this would be better to do as a publisher switch or author option than to hard-code it in. However, it's certainly quite useful to have around!