Support gzip for tile data

[jaakla]

For example tippecanoe exports tiles to directory as uncompressed pbf, but normally (like for mb studio) you need gzipped pbf, otherwise mb studio rejects it. The mbtiles metadata unfortunately does not specify tile data compression, so you kind of need to know whether "pbf" means plain or gzipped pbf. It has no much point for png or jpg as these are already compressed, but for pbf it makes a lot of sense - 50 and more % reduction.

So if you use mb-util to convert folder with pbf tiles to mbtiles the resulting mbtiles is big and just does not work for some targets. So I suggest to add flag -gzip to the tool to compress each tile data in the 'images' table. As a workaround manual compression of tiles before running mb-util (something like gzip *.pbf; mv *.pbf.gz *.pbf in each folder) is also an option, but with larger tilesets (I have biggest ones with ~100M tiles) it is kind of tedious.

[jaakla]

Submitted PR #96 with possible fix. Sorry, no test for it.

[e-n-f]

Tippecanoe should export compressed PBFs to directories unless you use --no-tile-compression, unless I got something wrong. (I'll check and make sure.) But thanks for the PR to manually add compression!

For symmetry, would you mind adding a --gunzip option to turn a compressed tileset into an uncompressed one?