Optional output parameter

I got quite annoyed with working with nota it would always overwrite the same file. We can do almost anything with it, but could prevent that. The bug made it vulnerable to strange issues with concurrency. Hence I quickly coded the addition of the `--output` paramater, which takes a path as argument. The file will be written there. In case the parameter is not supplied, the default `output.pdf` will be used

Fly-by fixes:

- There was an error in `README.md` (`--json` instead of `--data`). Fixed
- There was an old reference once the vagrant box was provisioned, so that _Quick Start_ did not work (`example.json` instead of `data.json)
- Slightly less memory for the vagrant box
- Some example fixes

@joostverdoorn could you verify my work?