Add feature id support

[jfirebaugh]

Fixes #59
Ports mapbox/geojson-vt@9ec3ea8

[mourner]

Looks good, pending fix of the Travis build.

[mourner]

Maybe alias std::experimental::optional<identifier> to optional_identifier, which is repeated a lot?

[jfirebaugh]

Build is failing due to linker errors that stem from this difference:

~/Development/geojson-vt-cpp $ .mason/mason ldflags glfw 3.1.2
-framework Cocoa -framework OpenGL -framework IOKit -framework CoreFoundation -framework CoreVideo
~/Development/geojson-vt-cpp $ .mason/mason ldflags glfw 3.2.1
-framework Cocoa -framework IOKit -framework CoreFoundation -framework CoreVideo

@springmeyer @kkaefer Do you know why -framework OpenGL is not appearing for glfw 3.2.1?

[kkaefer]

@jfirebaugh yeah, I recently debugged exactly that: GLFW only uses higher-level NS* objects, which are part of the Cocoa framework, and does not use any lower-level CGL* symbols. The OpenGL Framework is misleadingly named in that it contains the CGL implementation. The actual GL implementation is in libGL.dylib, which is also linked transitively through CoreVideo.

[jfirebaugh]

Ok, what does that mean for fixing the linker error?

[jfirebaugh]

@springmeyer Could you look into why the clang-3.8 build is failing?

[springmeyer]

@springmeyer Could you look into why the clang-3.8 build is failing?

Yes. @jfirebaugh latest mason only provides clang++ stable releases of 3.8.1 and 3.9.1 (https://github.com/mapbox/mason/blob/master/CHANGELOG.md#040). So, I'd recommend upgrading to 3.9. This is available in #61