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
```
`boolean` is a keyword. I think it should be quoted.