A base64url encoder and decoder.
It encodes and decodes using the base64url alphabet, decodes using both the base64 and base64url alphabets, does not allow invalid characters and requires padding.
Examples:
var encoded = BASE64URL.encode([0x62, 0x6c, 0xc3, 0xa5, 0x62, 0xc3, 0xa6,
0x72, 0x67, 0x72, 0xc3, 0xb8, 0x64]);
var decoded = BASE64URL.decode("YmzDpWLDpnJncsO4ZAo=");