Add process.stdin and out support so you can pipe data in and out of xp calls

[Marak]

We should be able to make calls like:

echo "foobar" | xp base64 

[tanepiper]

+1 would be good to be able to cat in files, both text and binary such as png files and be able to output as base64 for dataurl stuff.

[Marak]

Should be easy to add, I just didn't have time to squeeze it in the first release.