Allow equals sign to be omitted for named arguments

You currently have to use an equals sign when passing in a named argument:

```
python main.py my_command --desc="some message"
```

This can lead to confusion when people omit it, using this format instead:

```
python main.py my_command --desc "some message"
```

It would be good to support both formats.

Originally reported here: https://github.com/piccolo-orm/piccolo/issues/418