Add .nox directories to default exclude (#525)

[Nox](https://nox.readthedocs.io/) is similar to Tox. It creates a .nox directory that contains virtualenv for testing with different Python versions.

README.md

@@ -99,7 +99,7 @@ Options:
                               recursive searches. On Windows, use forward
                               slashes for directories.  [default:
                               build/|buck-out/|dist/|_build/|\.git/|\.hg/|
-                              \.mypy_cache/|\.tox/|\.venv/]
+                              \.mypy_cache/|\.nox/|\.tox/|\.venv/]
   -q, --quiet                 Don't emit non-error messages to stderr. Errors
                               are still emitted, silence those with
                               2>/dev/null.

black.py

@@ -50,7 +50,7 @@
 __version__ = "18.6b4"
 DEFAULT_LINE_LENGTH = 88
 DEFAULT_EXCLUDES = (
-    r"/(\.git|\.hg|\.mypy_cache|\.tox|\.venv|_build|buck-out|build|dist)/"
+    r"/(\.git|\.hg|\.mypy_cache|\.nox|\.tox|\.venv|_build|buck-out|build|dist)/"
 )
 DEFAULT_INCLUDES = r"\.pyi?$"
 CACHE_DIR = Path(user_cache_dir("black", version=__version__))