CLI: Add stricter parsing of repeated `--option value`

### Example

Pass an option `--foo` to configlet more than once. For example:

```
configlet sync --exercise=bob --exercise=two-fer
```

### Current behavior

Configlet uses the final option (here, `--exercise=two-fer`) and `--exercise=bob` is silently ignored.

### Possible behavior

We could show something like:

```text
Error: `--exercise` option can only be given once.
```

Alternatively, we could support passing multiple exercises via the syntax in the above example, or via something like:

```
configlet sync --exercise bob two-fer
```

or

```
configlet sync --exercise bob,two-fer
```