Fable bindings and helpers for React projects

One of the reasons not to add ReactLeaflet in fable-import was that repo only contains pure bindings which only have metadata and thus can be distributed just in .dll form. But ReactLeaflet contains actual code so it needs to include also sources in the distribution.

Adding Fable.ReactLeaflet to this repo makes it easier to maintain for us as a maintainer instead of having one repo per react package for example.