Add grib support

[ri0t]

There's probably few dozen different approaches to do this, but i currently favour the possibilities of combining the following:

• https://jswhit.github.io/pygrib/docs/index.html
• https://github.com/jwass/mplleaflet

[ri0t]

Not sure, the solution seems to be adding Vector data - if it can't deal with Grib data (pygrib seems to prefer producing bitmaps), using an imagelayer is another option: http://leafletjs.com/reference-1.0.2.html#imageoverlay