Support fragmentInURL option

[MateuszKubuszok]

Reveal.js supports fragmentInURL: true|false option which:

	// Flags whether to include the current fragment in the URL,
	// so that reloading brings you to the same fragment position
	fragmentInURL: false,

it is quite useful if one configure its gulp that on presentation.adoc it should generate presenation.html which in turn refreshes browser in (npm serve default behavior). This way, one can have real life preview of the presentation.

I found no way of passing such option from asciidoctor in the documentation, thus a feature request.

[a4z]

oh, interesting! since I use to write with life reload enabled, this seems to be exactly the feature that I miss.
this would spare me me workarround with using include files to always have just a hand full of slides active and quickly go to the edited one after reload
need to look this up

[a4z]

implementing this should be simple,
also need to research what pdfSeparateFragments attribute does, maybe that's related and should be added in the same PR.

when - sooner or later - I get feedback from @obilodeau on #204 and this is done, and I find the next time slot, I could have a look at this if on one else add it until than

[MateuszKubuszok]

@a4z I believe revealjs_history option mostly let you use this live preview workflow, but fragmentInURL should reinforce it, so that you should be able to navigate to exact fragment which would be even more awesome.

[obilodeau]

PR submitted see #214