Illegal return

Nobody has claimed this yet.

I'm running into a parsing issue (`Illegal return`) on [line 249](https://github.com/mapbox/dyno/blob/master/bin/cli.js#L249) of cli.js:

```
if (params.command === 'export') {
return dyno.describeTable(function(err, desc) {
if (err) {
console.error(err);
process.exit(1);
}

console.log(cleanDescription(desc));
scan();
});
}
```

I am pretty sure this is because `return` is being used outside of a function. This should probably be refactored.

cc @mapbox/atlas

Start in bin/cli.js at line 249 and inspect the surrounding command handling for the export branch. Refactor the control flow so the file parses without an Illegal return while preserving the described export behavior, then verify the CLI no longer reports the parsing error.

Written by the indexing model from the issue text.