add -r option

[Flonk]

I thought it would be nice to have the option to jump to a specific slide. I added the -r flag to add support for it.

[jessp01]

Looks good to me but I'm not sure -r is the best choice for this option (especially since r is used for reloading the input file when in presentation mode). How about -j?

[visit1985]

Thanks. Refactored and merged as -j.