using git urls for codemirror, showdown

`d-codemirror` and `d-showdown` are not on npm, so `npm install` fails.

This fix is pretty weak, but could actually be legitimate, I guess, if the intent of the examples is to be a bleeding edge test-bed.

Figured I'd close the loop on today's frustration, started over at [derby-templates](https://github.com/codeparty/derby-templates/issues/3), and continued in rediscovering this over at [derby-starter](https://github.com/codeparty/derby-starter/pull/4).