Add support for decoding base64url variant.

[wolfgang42]

This is a common style of base64 which swaps out + and / for URL/filename-safe equivalents.

Incidentally, should there be some checks for invalid characters in the decode function? I was trying to use base64-arraybuffer on some keys that used this encoding style and it was producing incorrect but plausible-looking results without any kind of error, which led to a very frustrating debugging session.

[Hexagon]

I've made a more complete implementation at #31 which later bacame @hexagon/base64