Add a support for the sf objects

[Nowosad]

The sf package is the R implementation of Simple Features and starts to be a new standard for working with spatial data in R. More information at https://github.com/edzer/sfr and http://robinlovelace.net/geocompr/spatial-class.html.

@hunzikp do you plan to add a support for the sf objects?

[hunzikp]

the development version of velox now supports sf objects.