Inject changed file into command

Allows users to programmatically use $FILE (or $EVENT) in their commands.

Such as:

```
watchy -w lib -- say "\$FILE changed"
```

Note that we escape the $FILE env value otherwise it gets evaluated immediately.

(Please excuse the typo in the actual commit, and the unsolicited PR!)
I think you want to copy everything from `process.env` first (and then add your `EVENT` and `FILE` - ideally only if they don't already exist).