Pass bufferSize param as a rendering option

After https://github.com/mapnik/node-mapnik/commit/5b823e0a52010e29a5d0c44adc590f9a28f217be, `tilelive-mapnik` lost the ability to render with a buffer.  This re-adds that ability, but instead of pulling from the `Map` object, it grabs the buffer size from `tilelive-mapnik`'s options.

(Probably warrants a new release to match mapnik@1.4.12+)

Fixes mapnik/mapnik#2378
Re: mapnik/node-mapnik#175