Simplify playground code and add snippets. (#136)

* Add instructions for how to run the playground locally.
* Make it easier to select backend in the elm code.
* Use scrolling for long scripts.
* Fix bug, send code to backend on connect, don't auto play on code changes.
* Compile and cache snippets.

.github/workflows/gh-pages.yml

@@ -82,7 +82,11 @@ jobs:
     - name: Playground
       run: |
         CWD=`pwd`
-        cd reanimate-playground/viewer-elm
+        stack build
+        cd reanimate-playground
+        stack build
+        stack exec --cwd ../ playground snippets reanimate-playground/snippets > viewer-elm/dist/snippets.js
+        cd viewer-elm
         npm install
         npm run build
         mkdir $CWD/hpc/playground