add `--force` flag - fixes #19

fixes #19

adds force flag to ignore missing files (but still show that it doesn't exist)
tests:

`node cli.js BADTHING || echo "err"`
```
Item doesn't exist
err
```
`node cli.js --force BADTHING || echo "err"`
```
Item doesn't exist
```
Nope:

```js
!true
//=> false
```

You're thinking of `+true` or `Number(true)`.