Add option to run hoodie from a folder other than the cwd

~~This is more a feature request than a pull request, because my implementation is just a quick hack.
I want to put this here for discussion though.~~

For the hoodie integration test setup I need to create a new hoodie app and start it from a subfolder, which is why I imagined a `childProcessOptions` option could work.
https://github.com/hoodiehq/hoodie-integration-test/tree/new

Known bugs in my implementation:
- [x] the data folder ist created in the original folder
- [x] Hoodie plugins aren't loaded as the original package.json seems to be used