Add support for base64url strings

[marvinruder]

The .base64() function validates strings that contain binary information encoded in Base 64. A modification of Base 64 that can safely be used in URLs and filenames is given by the base64url standard specified in RFC 4648 § 5. As summarized here,

it uses the same algorithm as the main standard, but differs in the following:

• Replaces “+” by “-” (minus)
• Replaces “/” by “_” (underline)
• Does not require a padding character
• Forbids line separators

I propose a method .base64url() that is similar to .base64(), but instead matches against a regular expression that describes base64url-encoded values.