Public channel directory names contain b'dir_name'

Run
```
python slack_export.py --token <my_token>
```

Expected: a directory is created with subdirectories named as 
```
public-channel1
public-channel2
private-channel
<etc.>
```

Observed: a directory is created with subdirectories named as 
```
b'public-channel1'
b'public-channel2'
private-channel
<etc.>
```