Add import/export markdown format from/to json format

[charphi]

Did you find some workaround? If not, maybe, "just" implementing a "toJson()" functionality (as clparse offers) would suffice to support the feature. (clparse -> json -> filter -> dif)

Originally posted by @koppor in #146 (comment)

clparse is another command line tool written in Rust that handles keep-a-changelog format.
It has the ability to export a markown into a json format that can be easily used in scripts.

Having this feature in heylogs would allow more integration with other tools