Blazing-fast tile merges!

Makes tile merging blazingly fast, as discussed in #49. The improvement is huge!

```
# zoom 9, without merge
scan russia polygon multizoom - z9 - z9 x 75.23 ops/sec ±2.73% (17 runs sampled)

# zoom 0-9, old merge algorithm
scan russia polygon multizoom - z0 - z9 x 0.82 ops/sec ±5.50% (5 runs sampled)

# zoom 0-9, new merge algorithm
scan russia polygon multizoom - z0 - z9 x 55.80 ops/sec ±2.52% (19 runs sampled)

# zoom 8-9, new merge algorithm
scan russia polygon multizoom - z8 - z9 x 54.27 ops/sec ±2.62% (18 runs sampled)
```

That means **70 times faster** for indexing Russian border from z0 to z9. Moreover, merging z0 to z9 is actually a bit faster than z8 to z9! How cool is that? [Point proven](https://github.com/mapbox/tile-cover/issues/49#issuecomment-62962537). 

So how about replacing `min_zoom` option with something like `mergeTiles: true`, which always goes up to z0?