FtCLI new param --port=/tmp/foo.txt

Let's make it possible to run `FtCLI` with this param:

```
--port=/tmp/foo.txt
```

If port is not a number, it has to be treated as file name. If this file is present, we read the port number from there. If it's not present, we allocate a new random port and save it into the file.

The main reason for this is to make `FtCLI` more thread/process safe.
