Suggestion: Add send_compressed_data to btrbk.conf.example

Hi, and thanks for this interesting tool.

I was noticing in initial testing that btrbk was sending the full size stream of files that were significantly compressed on source. At first I tried changing the line items

```
#stream_compress no
#stream_compress_level default
#stream_compress_threads default
```

To try to get the same behavior as ```btrfs send --compressed-data``` , but of course these did not work. It was not until I started poking around in the source that I found "send_compressed_data" option, which was the config switch that I needed. Setting this to yes gave me the desired behavior, that the target does not need to re-compress the data.

So, I suggest adding this to your example configuration file, either commented out as many of yours are, or perhaps having it on by default?