Add feature type

[jfirebaugh]

Fixes #8

[jfirebaugh]

Anyone know how to work around this GCC error:

In file included from include/mapbox/geometry.hpp:10:0,
                 from tests/test.cpp:1:
include/mapbox/geometry/feature.hpp:21:19: error: declaration of ‘mapbox::geometry::feature<T>::geometry_type mapbox::geometry::feature<T>::geometry’ [-fpermissive]
     geometry_type geometry;
                   ^
In file included from include/mapbox/geometry.hpp:9:0,
                 from tests/test.cpp:1:
include/mapbox/geometry/geometry.hpp:30:8: error: changes meaning of ‘geometry’ from ‘struct mapbox::geometry::geometry<T>’ [-fpermissive]
 struct geometry : geometry_base<T>
        ^

According to this SO post, GCC might be correct to issue this warning. I would like to find a workaround rather than renaming the geometry member.