Decode allow re-using a pre-allocated buffer?

[jeffvandyke]

For repeated invocations with temporary data, there's an opportunity (not sure how great) for memory allocation savings if instead of always creating a new ArrayBuffer, the decode function allows an optional 2nd parameter for a reusable ArrayBuffer that is console.assert()'ed to be at least the length needed for decoding the base64 string.

Perhaps the decode function could return the number of bytes written instead of an ArrayBuffer if a buffer is provided - speculating usefulness for handling variable-byte input strings.

Would you be open to a PR for this?