Add :url-safe keyword parameter to base64-{encode,decode}.

There are two things that I'd like you get your input for this patch.

- Function naming
  Python provides this feature as a different function (urlsafe_b64encode) than the standard one (b64encode). I've convinced myself that the keyword parameter is more Gauche-like but I'm not 100% sure.

- Keyword naming
  This encoding is referred to as "Base 64 Encoding with URL and Filename Safe Alphabet". Colloquially it's called "websafe" or "urlsafe" base64, I believe. I chose "url-safe" as it's clearer than "web-safe".