Add config for linters

* Add .editorconfig with basic whitespace settings
* Add package.json with dev dependencies for Prettier, ESLint, and
  stylelint
* Add stylelint configuration
* Add Prettier and ESLint configuration

.editorconfig

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+# See http://editorconfig.org for format details and
+# http://editorconfig.org/#download for editor / IDE integration
+
+root = true
+
+[*]
+indent_style = space
+indent_size = 4
+insert_final_newline = true
+trim_trailing_whitespace = true
+end_of_line = lf
+charset = utf-8
+
+# Makefiles always use tabs for indentation
+[Makefile]
+indent_style = tab
+
+# We don't want to apply our defaults to third-party code or minified bundles:
+[**/{external,vendor}/**,**.min.{js,css}]
+indent_style = ignore
+indent_size = ignore

.eslintrc.yaml

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+root: true
+extends: eslint:recommended
+plugins:
+    - prettier
+rules:
+    indent:
+        # https://eslint.org/docs/4.0.0/rules/indent
+        # Match defaults & Crockford but enable indentation for switch statement cases:
+        - 2
+        - 4
+        - SwitchCase: 1
+env:
+    browser: true
+    es6: true

.prettierrc

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+singleQuote: true
+bracketSpacing: false

.stylelintrc.yaml

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+extends:
+    - stylelint-config-recommended
+    - stylelint-config-prettier
+rules:
+    indentation: 4

package.json

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+{
+  "devDependencies": {
+    "eslint": "^4.19.1",
+    "eslint-plugin-prettier": "^2.6.0",
+    "prettier": "^1.13.4",
+    "stylelint": "^9.2.1",
+    "stylelint-config-prettier": "^3.2.0",
+    "stylelint-config-recommended": "^2.1.0"
+  }
+}