Add MADR support

[snisnisniksonah]

This PR adds dedicated support for the Markdown Archicturual Decision Records by @koppor and @socadk. A user can now initialize the repository using adr init DIR madr. MADR not only uses a different template.md, but also a different status format (see adr/madr#2). This is necessary, because MADR requires date and status information only when the status is not "Accepted".

We would be happy if this was included in adr-tools as adr-tools eases working with ADRs.

[tangzhen]

@npryce any update for this pull request?

[npryce]

I'm sorry. I'm not keen on supporting multiple formats. It adds non-trival maintenance overhead to ADR tools, and imposes complication on other tools that process the ADRs managed by adr-tools -- e.g. projects' documentation build pipelines, architecture visualisation tools. etc. Those tools would also have to become aware of the different formats, and how they define links between ADRs.

[npryce]

I'd be happier with multiple formats if there was a way to add formats in a non-intrusive way. E.g. without adding conditional logic that branches on format, and without the implementations of different formats having to be in the same repo as adr-tools itself. That way, each other format could be maintained by people who want to use it.