I've used the stack snippets feature a couple of times today (and love it!) by answering a couple of Bootstrap questions. It would be useful to include Bootstrap as well as jQuery, D3.js and AngularJs as available external libraries.
As you've already indicated, you can use the
external library button to add any lib you'd like. You can also copy-paste CDN links into your HTML directly.
I have to go with Shog on this one. Ultimately we all have our library preferences for things like Stack Snippets, and while I wish I could dedicate time to including (and maintaining/keeping up with) 50 of them, I think that user scripts are very likely the best answer here. That way, you can have exactly what you want and customize Stack Snippets to be exactly what you're looking for!