.ipp implementation architecture

[mapsam]

Let's build in .ipp files for this starter in case users want to reference the code that way. Just like mapnik vector tile.

[springmeyer]

👍 - this can follow mapbox/cpp#12 (comment) as a guide.

[mapsam]

@springmeyer should we follow the .ipp naming convention or move to -impl.hpp?

[springmeyer]

-impl.hpp

^^ I prefer _impl.hpp. I was historically in favor of .ipp since I noticed boost was using that naming (http://julio.meroh.net/2013/12/header-files-c-ipp-files.html), but @tmpsantos felt that _impl.hpp is more frequently used and easier to reason about (since it says explicitly in the file name that it is an "implementation header"). So I'm now 👍 on _impl.hpp. The mapnik-vector-tile lib ended up with .ipp but I would be happy to rename to _impl.hpp