Return info from server during test

Can you add new feature in iperf3 that can make possible returns current stats to client by interval that choosen by --interval option?

Now:
```
Connecting to host 192.168.2.187, port 5201
[ 5] local 192.168.2.4 port 51383 connected to 192.168.2.187 port 5201
[ ID] Interval Transfer Bitrate Total Datagrams
[ 5] 0.00-1.00 sec 11.9 MBytes 99.9 Mbits/sec 8626
[ 5] 1.00-2.00 sec 11.9 MBytes 100 Mbits/sec 8633
[ 5] 2.00-3.00 sec 11.9 MBytes 100 Mbits/sec 8632
[ 5] 3.00-4.00 sec 11.9 MBytes 100 Mbits/sec 8633
[ 5] 4.00-5.00 sec 11.9 MBytes 100 Mbits/sec 8632
[ 5] 5.00-6.00 sec 11.9 MBytes 100 Mbits/sec 8633
[ 5] 6.00-7.00 sec 11.9 MBytes 100 Mbits/sec 8633
[ 5] 7.00-8.00 sec 11.9 MBytes 100 Mbits/sec 8632
[ 5] 8.00-9.00 sec 11.9 MBytes 100 Mbits/sec 8633
[ 5] 9.00-10.00 sec 11.9 MBytes 100 Mbits/sec 8632
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bitrate Jitter Lost/Total Datagrams
[ 5] 0.00-10.00 sec 119 MBytes 100 Mbits/sec 0.000 ms 0/86319 (0%) sender
[ 5] 0.00-10.00 sec 119 MBytes 100 Mbits/sec 0.205 ms 0/86319 (0%) receiver

Server output:
-----------------------------------------------------------
Server listening on 5201
-----------------------------------------------------------
Accepted connection from 192.168.2.4, port 38910
[ 5] local 192.168.2.187 port 5201 connected to 192.168.2.4 port 51383
[ ID] Interval Transfer Bitrate Jitter Lost/Total Datagrams
[ 5] 0.00-1.00 sec 11.5 MBytes 96.6 Mbits/sec 0.196 ms 0/8336 (0%)
[ 5] 1.00-2.00 sec 12.1 MBytes 102 Mbits/sec 0.180 ms 0/8779 (0%)
[ 5] 2.00-3.00 sec 12.1 MBytes 101 Mbits/sec 0.152 ms 0/8745 (0%)
[ 5] 3.00-4.00 sec 11.7 MBytes 98.4 Mbits/sec 0.208 ms 0/8492 (0%)
[ 5] 4.00-5.00 sec 12.1 MBytes 101 Mbits/sec 0.183 ms 0/8753 (0%)
[ 5] 5.00-6.00 sec 11.9 MBytes 99.7 Mbits/sec 0.200 ms 0/8607 (0%)
[ 5] 6.00-7.00 sec 12.0 MBytes 100 Mbits/sec 0.193 ms 0/8662 (0%)
[ 5] 7.00-8.00 sec 11.9 MBytes 100 Mbits/sec 0.163 ms 0/8636 (0%)
[ 5] 8.00-9.00 sec 11.9 MBytes 100 Mbits/sec 0.231 ms 0/8640 (0%)
[ 5] 9.00-10.00 sec 11.9 MBytes 100 Mbits/sec 0.178 ms 0/8642 (0%)
[ 5] 10.00-10.00 sec 38.2 KBytes 97.8 Mbits/sec 0.205 ms 0/27 (0%)
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bitrate Jitter Lost/Total Datagrams
[ 5] 0.00-10.00 sec 119 MBytes 100 Mbits/sec 0.205 ms 0/86319 (0%) receiver

iperf Done.

```

But I think that this will be usefull to parsing current result on long-term test:
```
Connecting to host 192.168.2.187, port 5201
[ 5] local 192.168.2.4 port 51383 connected to 192.168.2.187 port 5201
[ ID] Interval Transfer Bitrate Total Datagrams
[ 5] 0.00-1.00 sec 11.9 MBytes 99.9 Mbits/sec 8626
**[ 5] 0.00-1.00 sec 11.5 MBytes 96.6 Mbits/sec 0.196 ms 0/8336 (0%)**
[ 5] 1.00-2.00 sec 11.9 MBytes 100 Mbits/sec 8633
**[ 5] 1.00-2.00 sec 12.1 MBytes 102 Mbits/sec 0.180 ms 0/8779 (0%)**
[ 5] 2.00-3.00 sec 11.9 MBytes 100 Mbits/sec 8632
**[ 5] 2.00-3.00 sec 12.1 MBytes 101 Mbits/sec 0.152 ms 0/8745 (0%)**
[ 5] 3.00-4.00 sec 11.9 MBytes 100 Mbits/sec 8633
**[ 5] 3.00-4.00 sec 11.7 MBytes 98.4 Mbits/sec 0.208 ms 0/8492 (0%)**
[ 5] 4.00-5.00 sec 11.9 MBytes 100 Mbits/sec 8632
**[ 5] 4.00-5.00 sec 12.1 MBytes 101 Mbits/sec 0.183 ms 0/8753 (0%)**
[ 5] 5.00-6.00 sec 11.9 MBytes 100 Mbits/sec 8633
**[ 5] 5.00-6.00 sec 11.9 MBytes 99.7 Mbits/sec 0.200 ms 0/8607 (0%)**
[ 5] 6.00-7.00 sec 11.9 MBytes 100 Mbits/sec 8633
**[ 5] 6.00-7.00 sec 12.0 MBytes 100 Mbits/sec 0.193 ms 0/8662 (0%)**
[ 5] 7.00-8.00 sec 11.9 MBytes 100 Mbits/sec 8632
**[ 5] 7.00-8.00 sec 11.9 MBytes 100 Mbits/sec 0.163 ms 0/8636 (0%)**
[ 5] 8.00-9.00 sec 11.9 MBytes 100 Mbits/sec 8633
**[ 5] 8.00-9.00 sec 11.9 MBytes 100 Mbits/sec 0.231 ms 0/8640 (0%)**
[ 5] 9.00-10.00 sec 11.9 MBytes 100 Mbits/sec 8632
[ 5] 0.00-10.00 sec 119 MBytes 100 Mbits/sec 0.205 ms 0/86319 (0%) receiver
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bitrate Jitter Lost/Total Datagrams
[ 5] 0.00-10.00 sec 119 MBytes 100 Mbits/sec 0.000 ms 0/86319 (0%) sender
[ 5] 0.00-10.00 sec 119 MBytes 100 Mbits/sec 0.205 ms 0/86319 (0%) receiver
iperf Done.

```