VS Code Prettier plugin as default formatter

With VS Code version 1.33.x a notification will be shown when there are
multiple formatter found for the file type in the currently active
editor. Since Prettier (1) supports most languages and is the de-factor
standard tool the VS Code Prettier plugin (2) has been configured as
editor default.
When there are unsupported languages or languages like Go that come with
their own toolchain (`gofmt`, `goimports` etc.) the default can be
overridden through the languages-specific settings scopes.

References:
  (1)  https://prettier.io
  (2) https://marketplace.visualstudio.com/items?itemName=esbenp.prettier-vscode

Resolves GH-197

snowblocks/visual-studio-code/settings.json

@@ -12,6 +12,7 @@
   "editor.codeLens": true,
   "editor.colorDecorators": true,
   "editor.cursorBlinking": "phase",
+  "editor.defaultFormatter": "esbenp.prettier-vscode",
   "editor.emptySelectionClipboard": false,
   "editor.fontFamily": "'Source Code Pro', 'Hack'",
   "editor.fontSize": 20,