Beancount-ing

- Rename to `beancount-ing`

- Generate `Balance` directives for opening / closing dates (thanks [szabootibor])

- Handle duplicate "Währung" field name (thanks [codedump] for the heads-up)
- Allow optional ";Letztes Update: aktuell" line in the header (thanks [szabootibor])

- Support "Kategorie" field in the CSV downloads
- Enable support for Python 3.9
- Drop support for Python 3.5

- Update `extract` interface

- Support Python 3.8