Allow to upload gists for different languages in the same document

[lucharo]

Sometimes articles combine several languages (python and bash or R, etc) and I'll like the gist tag to be used in markdown cells so that any gists can be uploaded. gists are not always present in articles to be executed, sometimes a writer may just want to display how to do a certain setup for which you may need bash highlighting but the rest of the notebook is written in python