add io command

gets a stream of data units on stdin, pipes a stream of results to stdout, displays progress on stderr (like curl).

to be used like
```
cat data.json | crowdprocess io -p program.js > results.json
```

Bonus: map/reduce in a line ;)
```
cat data.json | crowdprocess io -p map.js | crowdprocess io -p reduce.js > results.json
```