RFC4648 specifies an alternate alphabet for base64 encoding / decoding
where '+' and '/' are exchanged for '-' and '-' to make the resulting
string safe to use in filenames and URLs.
This adds a base64url_encode() function that uses the alternate alphabet.
The base64_decode() function is extended to accept both alphabets.
- suppressed cppcheck warning for `unassignedVariable` for
the encode/decode arrays in stream encode/decode tests
as these arrays are assigned by the according `base64_[en|de]code()` function.
