parse nginx log_format

Right now rhit assumes the log format is the default one, which is quite common.

Among the reasons I had not to interpret the configured log format:

- it's painful to write an efficient enough variable parser
- changing the configuration means the past isn't readable anymore, so it can't be perfect
- the log_format variable lets you define unparsable and ambiguous log lines
- you may define formats missing essential fields

So, as long as it looks like I can have something working for 99% of users, using a few tricks and dynamic recognitions, I won't implement this issue.

Nevertheless, I'm not sure not parsing the `log_format` will always be OK, hence this issue.