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

Close #79
Close #80

refactor: simplify ephemerides, remove dead code

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

feat: add support for relative dates on --date argument

ci: run the tests in the PRs

fix: Fix wording in the PDF output

chore(deps): bump skyfield from 1.17 to 1.18

Add support for occultations

Bumps [skyfield](https://github.com/brandon-rhodes/python-skyfield) from 1.17 to 1.18.
- [Release notes](https://github.com/brandon-rhodes/python-skyfield/releases)
- [Changelog](https://github.com/skyfielders/python-skyfield/blob/master/CHANGELOG.rst)
- [Commits](https://github.com/brandon-rhodes/python-skyfield/compare/1.17...1.18)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Add support for environment variables for the observer's position and timezone

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

chore(deps-dev): bump codecov from 2.0.16 to 2.0.22

chore(deps): bump tabulate from 0.8.6 to 0.8.7

chore(deps): bump numpy from 1.18.1 to 1.18.2

Bumps [codecov](https://github.com/codecov/codecov-python) from 2.0.16 to 2.0.22.
- [Release notes](https://github.com/codecov/codecov-python/releases)
- [Changelog](https://github.com/codecov/codecov-python/blob/v2.0.22/CHANGELOG.md)
- [Commits](https://github.com/codecov/codecov-python/compare/v2.0.16...v2.0.22)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [tabulate](https://github.com/astanin/python-tabulate) from 0.8.6 to 0.8.7.
- [Release notes](https://github.com/astanin/python-tabulate/releases)
- [Changelog](https://github.com/astanin/python-tabulate/blob/master/CHANGELOG)
- [Commits](https://github.com/astanin/python-tabulate/compare/v0.8.6...v0.8.7)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Bumps [numpy](https://github.com/numpy/numpy) from 1.18.1 to 1.18.2.
- [Release notes](https://github.com/numpy/numpy/releases)
- [Changelog](https://github.com/numpy/numpy/blob/master/doc/HOWTO_RELEASE.rst.txt)
- [Commits](https://github.com/numpy/numpy/compare/v1.18.1...v1.18.2)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

fix: remove Mercury and Venus opposition

Add support for Moon conjunctions

Prevent false detection of conjunctions

fix: remove the "timezone" word in the PDF output’s introduction

Handle KeyboardInterrupt exception

Using Ctrl+C to interrupt the program won't display a big ugly error
anymore.

feat: move the date to a single argument

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

chore(deps-dev): bump codecov from 2.0.15 to 2.0.16

Bumps [codecov](https://github.com/codecov/codecov-python) from 2.0.15 to 2.0.16.
- [Release notes](https://github.com/codecov/codecov-python/releases)
- [Changelog](https://github.com/codecov/codecov-python/blob/master/CHANGELOG.md)
- [Commits](https://github.com/codecov/codecov-python/compare/v2.0.15...v2.0.16)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Update doc, add man page