Add IPFS support in markdown parsers

[markg85]

Support can be in 2 levels:

1. When you explicitly add a link (like [name](url)) properly handle IPFS as links too.
2. In text, detect CID's en "linkify" them.

The markdown-it library seems popular and should probably be a first candidate to implement this in.
Don't know where normal link parsing would fit in.
Detecting it in text would be in the linkify-it plugin