Way to drop certain columns

[ljbade]

I was struggling with gid and fid columns bloating the size of my vector tiles.

@springmeyer suggested editing start_tile_feature to add support to drop certain columns during encoding.

E.g.

void start_tile_feature(mapnik::feature_impl const& feature)

to

void start_tile_feature(mapnik::feature_impl const& feature, std::vector<std::string> const& fields_to_exclude)

[ljbade]

I will take a stab at this.