Add support for markdown files

[lmihalkovic]

Follow-up to #4197, it would be nice to allow READMEs written in markdown to display with all enhancements

[ffissore]

See #4197 (comment)

[lmihalkovic]

I know, Arduino Create will do it. It is quite unfortunate that the desktop IDE is headed to become the ugly girlfriend... the code is not always pretty (you know it better than me), but it is still salvageable.

[LMDONE]