CloudFront / Extended Log Format support

The extended log format has variable header fields, however they must be specified in the header.

```
#Software: Microsoft Internet Information Services 6.0
#Version: 1.0
#Date: 2002-05-24 20:18:01
#Fields: date time c-ip cs-username s-ip s-port cs-method cs-uri-stem cs-uri-query sc-status sc-bytes cs-bytes time-taken cs(User-Agent) cs(Referrer)
2002-05-24 20:18:01 172.224.24.114 - 206.73.118.24 80 GET /Default.htm - 200 7930 248 31 Mozilla/4.0+(compatible;+MSIE+5.01;+Windows+2000+Server) http://64.224.24.114/
```

References:
- https://www.microsoft.com/technet/prodtechnol/WindowsServer2003/Library/IIS/ffdd7079-47be-4277-921f-7a3a6e610dcb.mspx?mfr=true
- https://www.microsoft.com/technet/prodtechnol/WindowsServer2003/Library/IIS/676400bc-8969-4aa7-851a-9319490a9bbb.mspx?mfr=true
- http://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/AccessLogs.html#BasicDistributionFileFormat
- http://www.w3.org/TR/WD-logfile.html

Problems:
- Variable fields
- Output may want to specify different headers