Add auto-formatting via black

Introduce black, a Python code formatter. Also a Makefile that
includes two common targets:

* fmt (auto-format the code)
* lint (lint the code and return non-zero if out of spec)

We can re-use the latter in CI.

By introducing a code formatter, we can cut down on stylistic
variations across a now-large codebase. We can also avoid most style
discussions in PR if we just have this thing enforce it for us.

Makefile

@@ -0,0 +1,12 @@
+default: install
+
+BLACK_FORMAT_CONFIGS = --target-version py37 --line-length 100
+
+install:
+	python setup.py develop
+
+fmt:
+	black $(BLACK_FORMAT_CONFIGS) evennia
+
+lint:
+	black --check $(BLACK_FORMAT_CONFIGS) evennia