Add warning when passing positional parameters to commands that don't accept them

Steps to reproduce:

```
$ wp core download /tmp/wp-cli-test
```

Expected:

```
Warning: Too many positional parameters: /tmp/wp-cli-test
Success: WordPress downloaded.
```

Which would at least give me a clue that something is wrong. The correct command is:

```
$ wp core download --path=/tmp/wp-cli-test
```

Related: #435