* feat: replace latitude and longitude with one argument alone: --position

* feat: add support for Plus Code

From Wikipedia: “The Open Location Code is a geocode system for
identifying an area anywhere on the Earth.”
A Plus Code has a the form of small alphanumerical codes like: `9F25J3H5+M8`

* docs: update the manpage

Show a graph instead of a table for the ephemerides in 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: the --day, --month and --year arguments have been removed in the
favor of a single one.