The syntax for pg.set_config is awkward

Because of the way fabric's parsing works, if you want to include a comma you must escape it with a backslash, like so:

```
# this works
fab pg.set_config:shared_preload_libraries,'citus\,cstore_fdw'
# you have to escape it even if you use keyword args (the below does not work)
fab pg.set_config:shared_preload_libraries='citus\,cstore_fdw'
```

We could monkeypatch the parsing to make it something more reasonable.