Running s3backup on a schedule

**Is your feature request related to a problem? Please describe.**
At the moment, you have to run s3backup manually. It would be nice if once you installed the tool, it would run itself.

**Describe the solution you'd like**
Something to trigger s3backup automatically

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
Some additional considerations:
* How would this work across different platforms?
* Does it need to be integrated in to an insaller? See #4

Start by reviewing how the s3backup command is invoked and the installer work referenced in issue #4. Compare scheduling options across the platforms the tool supports and define whether installation should configure them; done means an agreed, platform-specific way to trigger s3backup automatically.

Written by the indexing model from the issue text.