Addition of on delete actions for overlays

[LyndonArmitage]

(FYI: I am not really an angular or JavaScript developer, having not done any proper front end development in a long while so apologies if this doesn't fit the code style etc.)

This is partially addressing the TODO present in the layers directive:

TODO: Depending on the layer type we will have to delete what's included on it

We noticed on one of our applications that there was a memory leak to do with a heatmap overlay that refreshed every 10 seconds. It seemed like Chrome was keeping a reference to the many points being fed into the layer, adding in the ability to clean up the heatmap layer upon refresh seemed to eliminate this issue.

I might be able to come up with an example piece of code showing the issue at some point but basically this PR adds the ability to add an action that should be executed on the old layer before it is removed from the leaflet overlays array.

Example configuration (using heatmap support provided by ui-leaflet-layers):

      {
        heat: {
          name: 'Heat Map',
          type: 'heat',
          data: [],
          layerOptions: {
            radius: 15,
            blur: 15,
            minOpacity: .4,
            max: 1,
            gradient: defaultGradient,
          },
          visible: true,
          doRefresh: false,
          cleanupAction: function (layer) {
            console.log("cleaning up layer");
            layer.setLatLngs([]);
          }
        },
      }