Add format decoding pipelines

[vearutop]

Some values can have particular encoding that can be decoded, this can be helpful to safely extract specific pieces of data.
We can have a configurable mapping of key: format to use during flattening.

{
  "decodeKeysRegex": {
         ".foo.*.*": "url",
         ".foo.*.*.URL.query.bar": "base64",       
  }
}

[vearutop]

#53