Enable CircleCI (observablehq#91)

Author: tmcw

.circleci/config.yml

@@ -0,0 +1,16 @@
+version: 2
+jobs:
+  build:
+    docker:
+      - image: circleci/node:10.14.0
+    steps:
+      - run:
+          name: yarn
+          command: curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.9.4
+      - checkout
+      - run:
+          name: install
+          command: $HOME/.yarn/bin/yarn --frozen-lockfile
+      - run:
+          name: test
+          command: $HOME/.yarn/bin/yarn test