Add area select "plugin"

[cryogenian]

Basically this task stands for making leaflet-areaselect functionality here.

Motivation here is following

• Leaflet guys use global variables and just include their plugins into HTML.
• Porting unknown API to PS has almost the same complexety as reimplementing it directly.

Here is some things needed to do:

• Control switching to edit mode and back, also hides, shows selected areas.
• Options for multiple/single select, color.
• Add/remove/edit rectangles. Somehow mark them to avoid editing other vector layers.
• Extract lat/lng and all vector/raster layers that overlaps with them
• Make pre-eventsource for latter use in purescript-halogen-leaflet raising events on editing/adding/removing rectangles.