http: JSON format access logging support

[hongzhidao]

Example:

    "access_log": {
        "path": "/tmp/access.log",
        "format": {
            "remote_addr": "$remote_addr",
            "time_local": "$time_local",
            "request_line": "$request_line",
            "status": "$status",
            "body_bytes_sent": "$body_bytes_sent",
            "header_referer": "$header_referer",
            "header_user_agent": "$header_user_agent"
        }
    }