feature: custom timeperiod (#3)

+ add ability to define a custom time period

This feature enables users to further narrow the expected query result by customizing the time period.
Every script feature is able to handle the custom timestamps, but beware no further consistency checks are done. Thus if you provide a stop timestamp that is prior to the start timestamp the output will be empty, no error whatsoever.

Timestamps must be conformal to the Python [datetime](https://docs.python.org/3/library/datetime.html#strftime-strptime-behavior) module. The easiest way to achieve that is to use the ISO-8601 format from the unix date function:

```bash
$date -Ins -d "3 weeks ago"
$date --iso-8601 -d "3 weeks ago"
```

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