Add do_compression option

When I try to use that function, number of tiles was reduced by half but size of database increases!

```bash
./mb-util --scheme=tms ~/coding/tiles_page/gits/ gits.mbtiles
./mb-util --do_compression --scheme=tms ~/coding/tiles_page/gits/ gits_compressed.mbtiles

sqlite3 gits_compressed_integer.mbtiles 'select count(*) from images;'
100818

sqlite3 gits.mbtiles 'select count(*) from tiles;'
200747

ls -sh | grep gits
264M gits.mbtiles
270M gits_compressed.mbtiles
```

Then I replaced type of field tile_id from string (uuid) to integer (aka regular foreign key) and I got following results:

```bash
ls -sh | grep gits
264M gits.mbtiles
270M gits_compressed.mbtiles
256M gits_compressed_integer.mbtiles
```

Is there any reason to use uuid instead of simple integer?

https://github.com/mapbox/mbutil/issues/38, https://github.com/mapbox/mbutil/issues/39, https://github.com/mapbox/mbutil/issues/40