Add AppendSkip method that does not allocate new slice

[ernado]

We have

// SkipAndReturnBytes skip next JSON element, and return its content as []byte.
// The []byte can be kept, it is a copy of data.
func (iter *Iterator) SkipAndReturnBytes() []byte {
	iter.startCapture(iter.head)
	iter.Skip()
	return
}

I propose something like that:

// AppendSkip skips next JSON element and appends its content to
// buffer, returning the result.
func (iter *Iterator) AppendSkip(buf []byte) []byte {
	return append(buf, iter.SkipAndReturnBytes()...)
}

Obviously, the internal implementation will differ and will not allocate a new slice.
It is like Time.AppendFormat, but for SkipAndReturnBytes.

This will allow allocation optimizations in case of concurrent parsing.

If you are interested in such implementation, I can prepare a PR.