Add missing Forecast.io weather icons

[chrisgriffith]

To create parity with Forecast.io's icon names, three additional icons are needed:
Sleet
Wind
Fog

[bensperry]

Good idea. Will add to the next (v4) release!

[lowlevelm]

It would also be nice to have:

• hail
• tornado

[saeedesmaili]

The weather-related icons (like wind) haven't been added yet, right?

[christian-bromann]

Consolidating into #1449