Added 'base64url' mapping to choose from...

[PaulWay]

Added 'base64url' mapping to choose from (using not yet provided 'choose map' mechanism).

[PaulWay]

BTW, the base64_maps_t struct repeats itself, and there would be hilarious consequences if a PEBKAC caused the forward map to be different from the reverse map. I generally prefer to take the forward map and generate the reverse map during initialisation (or some other process). Is it worth doing that? I realise that takes away the 'const' nature of the structure...