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 cwd option could work.
https://github.com/hoodiehq/hoodie-integration-test/tree/new

Known bugs in my implementation:
1. the data folder ist created in the original folder
2. Hoodie plugins aren't loaded as the original package.json seems to be used