Extend batch to take a function parameter

An extension of `batch` with a function parameter that's used to determine when to push out the current batch, rather than pushing it out every `n` elements.

The idea comes from this split-related example:
https://github.com/caolan/highland/issues/6#issuecomment-44143123

Start by locating Highland's implementation and tests for `batch`, then read the linked split-related example for the intended behavior. Define how the function parameter determines when the current batch is emitted, and update the relevant tests so the new behavior and existing fixed-size batching are covered.

Written by the indexing model from the issue text.