Add Cosine distance.

```R
# Default distance is Euclid.
bigkmeans(x, centers, iter.max = 10, nstart = 1)
bigkmeans(x, centers, iter.max = 10, nstart = 1, dist="euclid")

# Cosine distance.
bigkmeans(x, centers, iter.max = 10, nstart = 1, dist="cosine")
```