Slf string key generator additions

    Add base64, concat_binary, and as-is-binary key generators
    
    New key generators:
    
    * {base64, BinaryGenerator} will Base64-encode the binary created
      by BinaryGenerator.
    * {concat_binary, GenOne, GenTwo} will concatenate the binaries
      created by the GenOne and GenTwo generators.
    * Any literal binary B will create a fixed binary generator that
      always generates B.

Also, add a sample config file that uses these new generators.