zoom-slider: fix css; refactor
zoom-slider: fix css; refactor

* add css to align with other leaflet controls
* add changeable pan control `options` to plugin namespace
* fix try/catch block
* simplify plugin structure

McBen/ingress-intel-total-conversion,3ch01c/ingress-intel-total-conversion,sndirsch/ingress-intel-total-conversion,3ch01c/ingress-intel-total-conversion,sndirsch/ingress-intel-total-conversion,sndirsch/ingress-intel-total-conversion,3ch01c/ingress-intel-total-conversion,3ch01c/ingress-intel-total-conversion,3ch01c/ingress-intel-total-conversion,sndirsch/ingress-intel-total-conversion,3ch01c/ingress-intel-total-conversion,McBen/ingress-intel-total-conversion,McBen/ingress-intel-total-conversion,McBen/ingress-intel-total-conversion,sndirsch/ingress-intel-total-conversion,McBen/ingress-intel-total-conversion