Polipo: introduce --with-large-chunks for 16k size

Polipo FAQ mentions the CHUNK_SIZE compile-time option:

* it defaults to 8k
* to lower the performance and RAM usage, one could lower it up to 2k
* to increase the performance and RAM usage, one could take it up to 16k

It seems like the `--chunk-size` option in bytes is complicated and the only reasonable usage of this is raising the CHUNK_SIZE to 16k.

So, I'm introducing a `--with-large-chunks` option to Polipo that will make it so. Hope, that helps somebody.