guess output format based on output file extension

check for empty output file

default to .txt if no file extension provided

change default output format back to txt

add tests

update man page

fix default output format

directly return output dumper

use keys from dumpers in key error message

fix man pages

prevent output format from being overridden

use first dumper key as default format

update messages and formatting

update help message for format argument

specify list type in InvalidOutputFormat exception

make default output format more apparent

update messages

* 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

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

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.