Optional parameters in decode/encode

Would be nice to have an official recommendation on optional parameters:
- should they be discouraged
- should they be after `offset` in `encode` and `end` in `decode`
- any other option?

I'm thinking about following `abstract-encoding` in https://github.com/ethereumjs/rlp, which has:

> rlp.decode(encoded, [skipRemainderCheck=false]) - Decodes a RLP encoded Buffer, Array or String and returns a Buffer or an Array of Buffers. If skipRemainderCheck is enabled rlp will just decode the first rlp sequence in the buffer. By default it would through an error if there is more bytes in Buffer than used by rlp sequence.

No files, tests, or entry points are identified in the issue. Review the current encode and decode signatures and documentation, then compare them with the referenced abstract-encoding and ethereumjs/rlp APIs; done means an agreed recommendation for optional-parameter placement and whether such parameters should be supported.

Written by the indexing model from the issue text.