Add pre-commit hooks to ensure code formatting

.pre-commit-config.yaml

@@ -0,0 +1,8 @@
+repos:
+-   repo: https://github.com/ambv/black
+    rev: 18.9b0
+    hooks:
+    - id: black
+      language_version: python3.6
+      language: system
+      entry: python -m black

environment.yml

@@ -4,6 +4,7 @@ channels:
 dependencies:
   - python=3.6
   - attrs
+  - pre-commit
   - autopep8
   - black
   - click