[includedeck] allow to add some theme name in the url to experiment with themes

* example .../samples/deck-smartsyntax.html##web-2.0
* example .../samples/deck-smartsyntax.html#slide-23#swiss
* needs a strong refresh (shift refresh or loading the page from a new tab)
* works only when the presentation uses one of the default theme with includedeck (theme:...)