Investigate use of eval to allow a callback to the current bash shell

Might look something like this:

```
export PS1="\$(eval \$(/usr/local/bin/prompty \$?))"
```

Prompty would then output something like:

```
export SOMETHING=true; echo -n "*prompt*"
```

Would this allow for data to persist across prompty calls from the same shell session? Only one way to find out!

http://stackoverflow.com/questions/716011/why-cant-environmental-variables-set-in-python-persist
