Introduce ``below`` option

We have two conflicting expectations regarding z-index of text.

See:
* https://github.com/makinacorpus/Leaflet.TextPath/issues/20
* https://github.com/makinacorpus/Leaflet.TextPath/issues/5

And respective jsfiddle:
* http://jsfiddle.net/sir_kitty/cmhbQ/2/
* http://jsfiddle.net/5d3owLdw/3/

@Abbe98 @fuatsengul @akshayme @p-j @solo999 do you guyz have a better idea that adding an option like this ?

Thanks for your feedback !