Add pre-commit configuration (closes #223)

.pre-commit-config.yaml

+# See https://pre-commit.com for more information
+# See https://pre-commit.com/hooks.html for more hooks
+repos:
+-   repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: v4.0.1
+    hooks:
+    -   id: trailing-whitespace
+    -   id: end-of-file-fixer
+    -   id: check-yaml
+    -   id: check-added-large-files
+-   repo: https://github.com/psf/black
+    rev: 21.6b0
+    hooks:
+    -   id: black
+-   repo: https://gitlab.com/pycqa/flake8
+    rev: 3.9.2
+    hooks:
+    -   id: flake8
+        args: [--config=./.flake8]