Don't run service if vdirsyncer is not configured

If vdirsyncer is not configured, simply exit with non-failure, since
there's nothing to do.

Don't say vdirsyncer was successful if it wasn't

Configure pre-commit hook for linting

systemd: This is a background service

Use constants for predefined intervals

Make the config reusable as a library

Add installation and usage instructions

Set up builds and tests on CI

Fix linting issues spotted by staticcheck

Clarify this caveat

Typo in README

Get storage paths from vdirsyncer

Remove hardcoded storage paths, and use those configured in vdirsyncer.
Note: this uses the `vdirsyncer showconfig` command, which is not
release in a stable version yet.

aur: Add make target to push updates

aur: include vdirsyncer as a dependency

Include an ArchLinux package

List some pending improvements

Also install the man page

Initial commit