Add plugin to move zoom control

[adamdsmith]

I often find myself wanting to relocate the zoom control from the default "upperleft". This function does that. I suspect there's a more preferred means of adding this functionality rather than the "plugin" approach I've taken.

library(leaflet.extras)

# Place zoom control at bottom right by default
leaflet() %>%
    addTiles() %>%
    moveZoomControl()

# Other standard location references work as well
leaflet() %>%
    addTiles() %>%
    moveZoomControl("bottomleft")

[bhaskarvk]

I'll be resuming development of leaflet.extras in Dec and will tackle this in that round. Thanks!

[trafficonese]

Could you maybe reopen a new and clean PR and also include the installation method for the plugin? Is it available via npm or is it only a github repo?