+ allow processing logs in json

+ allow processing logs in json

Nowadays in many cases it is more convenient to have logs in json. These
changes allow process such logs without losing any previous
functionality.

The new option is introduced - `LogFormatJsonMap`. This option should
contain a json for mapping keys in log file into awstats's predefined
keys.  This configuration will be used only if the `LogFormat` will have
the value `json`.
Example:
```
LogFormat=json
LogFormatJsonMap={"host":"remote_addr","logname":"remote_user","time1":"time","method":"request_method","url":"request_uri","code":"status","size":"bytes_sent","referer":"http_referer","agent":"http_user_agent"}
```

NB. The are no direct correspondence of predefined keys and `$pos_*`
variables. So `LogFormatJsonMap` should use suffixes from `$pos_*`
variables. As example: instead of `bytesd` the key `size` should be
used; instead of `ua` - `agent`.