[Lint] Add pre-commit config

- Added a pre-commit config for code linting and formatting. It will
  auto-format python, javascript, CSS, YAML and markdown files to save
  manually doing so. To install:

      pip install pre-commit
      pre-commit install

- Added a default virtual environment directory to gitignore.

.gitignore

@@ -17,3 +17,4 @@ deluge.pot
 .build_data*
 osx/app
 RELEASE-VERSION
+.venv

.pre-commit-config.yaml

@@ -0,0 +1,38 @@
+repos:
+  - repo: https://github.com/ambv/black
+    rev: 18.9b0
+    hooks:
+      - id: black
+        name: Fmt Black
+        language_version: python3.6
+  - repo: https://github.com/prettier/prettier
+    rev: 1.14.3
+    hooks:
+      - id: prettier
+        name: Fmt Prettier
+        files: "\\.(
+          css\
+          |less\
+          |scss\
+          |html\
+          |ts|tsx\
+          |graphql|gql\
+          |json\
+          |js|jsx\
+          |md|markdown|mdown|mkdn\
+          |yaml|yml\
+          )$"
+  - repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: v1.4.0
+    hooks:
+      - id: double-quote-string-fixer
+        name: Fix Double-quotes
+      - id: end-of-file-fixer
+        name: Fix End-of-files
+      - id: mixed-line-ending
+        name: Fix Line-endings
+        args: [--fix=auto]
+      - id: flake8
+        name: Chk Flake8
+        additional_dependencies:
+          - flake8-isort==2.5

setup.cfg

@@ -23,8 +23,8 @@ known_third_party =
     bbfreeze, win32verstamp,
 # Ignore gtk modules, primarily for tox testing.
     pygtk, gtk, gobject, gtk.gdk, pango, cairo, pangocairo,
-# Ignore sphinx dependencies.
-    recommonmark,
+# Ignore other module dependencies for pre-commit isort.
+    twisted, OpenSSL, pytest, recommonmark, chardet, pkg_resources, zope, mock
 known_first_party = msgfmt, deluge
 order_by_type = true
 not_skip = __init__.py