Idea: for include links use file extension as language by default

[tad-lispy]

Hi! Thank you for this program.

I just started using it (via excellent devenv) and it looks super useful to me when preparing learning materials. Especially the include link feature. One thing that sticks out though is that to have syntax highlighting I need to write the language explicitly after the link. For example:

[$ sync-io.js](./sync-io.js) as js

It looks a bit unnatural and I think shouldn't be necessary. After all the language is implied from the linked file extension. Would it make sense to make the as js part optional, and in it's absence use the file extension as language in the resulting block?

[zimbatm]

Yes, that's a good idea actually

[zimbatm]

The language recognized by the syntax highlighter doesn't alwyas match the file extension, but that's already going in the right direction.