Add markdown support

[AlexNi245]

The previous version of Dm3 has supported Markdown input. The current version does not.
Please check how markdown was supported in the old version and add it to the current version.

it might help to travel back in time to see how it was solved back then https://github.com/dm3-org/dm3/tree/dc91ee7538c734edded2eca79e561186e4f46e9a