Remove trailing whitespace from generated config file

When `answers` object contains empty arrays for `FILES`, `EXCLUDE` or `PREPROCESSORS` it generates lines with trailing whitespace which can be annoying.
this commit changes templates and processing functions

before:
```javascript
    // list of files / patterns to load in the browser
    files: [
______      
    ],
```

now:
```
    // list of files / patterns to load in the browser
    files: [
    ],
```