BREAKING CHANGE: on Ubuntu 20.04+, the `texlive-latex-extra` package is needed to generate the PDF output

The observer's position and timezone can now be set using the following
environment variables:

- `KOSMORRO_LATITUDE`: the observer's latitude
- `KOSMORRO_LONGITUDE`: the observer's longitude
- `KOSMORRO_TIMEZONE`: the observer's timezone

BREAKING CHANGE: JSON format now returns the moon phase as an object
instead of a string