bring your own buffer and byteOffset

Looking at `protocol-buffers` i noticed that the API supports [a `buffer` and `offset`](https://github.com/mafintosh/protocol-buffers/blob/a742b1ea638f6b5e6b7f9dcb37220a80f8a90a38/compile.js#L149) property in the `encode` function and [a `offset` and `end` option](https://github.com/mafintosh/protocol-buffers/blob/a742b1ea638f6b5e6b7f9dcb37220a80f8a90a38/compile.js#L404) in the decode function and I have been wondering if it would be reasonable to implement these options also in codecs to have API compatibility between the two libraries.

Compare the linked protocol-buffers compile.js encode and decode behavior with the codecs package's encode/decode entry points. Determine how buffer, offset, and end should behave across the codecs, then verify that the resulting API is compatible with protocol-buffers.

Written by the indexing model from the issue text.