Show a graph instead of a table for the ephemerides in the PDF output

BREAKING CHANGE: the JSON format has deeply changed to enhance its
consistency

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: the --day, --month and --year arguments have been removed in the
favor of a single one.

You can now use the `--format=pdf` argument to save the data computed by
Kosmorro to a PDF file. The new `--output` argument must be given with
the export file path.

Colors can be disabled with the new `--no-colors` argument.