Allow encryption of multiple variables

This allows you to do something like the following:

```
$ travis encrypt <<EOF
username=random
password=random2
EOF
Please add the following to your .travis.yml file:

  secure: "gfs4uN5PQcp..."
  secure: "asdfsdfDF..."

Pro Tip™: You can add it automatically by running with --add.
```

Requested in #4