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
```
They both do the same thing. `!` coerces it into a boolean. It will always be `true` or `false`. Does this seem hard to read to you? I find `test ? 0 : 1` harder to read.