WAT extractor: missing WARC format version

[saraaubry]

In the envelope structure, the Format metadata field is missing the version number. It will give the possibility to keep track of WARC format evolutions overtime.

Current:
Envelope: {
Format: "WARC",
}

Target:
Envelope: {
Format: "WARC/1.0", (stated at the top of the named fields on individual records)
}