Added render utility

Changed the `avecado` command so that it can either make vector tiles or render raster tiles.
This error message could triggered by a problem with `mapnik::load_map`, `avecado::make_tilejson_fetcher`, `(*fetcher)(z, x, y).get()`, `avecado::render_vector_tile` or `mapnik::save_to_file`. It's not always obvious from `e.what()` where the error is coming from