allow change of font-size of icon fonts

[rriemann]

I would like to use the font-awesome icons, which supply a special class 'fa-lg' to get 33% bigger icons. Unfortunately this doesn't work, because the css of awesome-markers overwrites the font-size afterwards with a statical value:

https://github.com/lvoogdt/Leaflet.awesome-markers/blob/af0bfc870cae53eb0bd18b84849fe65998e33667/dist/leaflet.awesome-markers.css#L42

Would you please consider to make the font-size adjustable? font-awesome icons seems to be a bit smaller and I would like to tune that a little bit.