Add EditorConfig (#1119)

* Add EditorConfig file.

This allows users with a wide variety of editors to easily code in
OpenZeppelin's preferred 2 space indentation code style.

See https://editorconfig.org for more information.

* Eslint: Always disallow trailing space

* Eslint: Error on missing EOL at file end

.editorconfig

+# EditorConfig is awesome: https://EditorConfig.org
+
+# top-most EditorConfig file
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+indent_size = 2
+indent_style = space
+insert_final_newline = true
+trim_trailing_whitespace = true

.eslintrc

@@ -28,7 +28,7 @@
     "comma-dangle": ["warn", "always-multiline"],
     "comma-spacing": ["error", {"before": false, "after": true}],
     "dot-notation": ["error", {"allowKeywords": true, "allowPattern": ""}],
-    "eol-last": "warn",
+    "eol-last": ["error", "always"],
     "eqeqeq": ["error", "smart"],
     "generator-star-spacing": ["error", "before"],
     "indent": ["error", 2],
@@ -38,7 +38,7 @@
     "no-dupe-keys": "error",
     "no-mixed-spaces-and-tabs": ["error", "smart-tabs"],
     "no-redeclare": ["error", {"builtinGlobals": true}],
-    "no-trailing-spaces": ["error", { "skipBlankLines": true }],
+    "no-trailing-spaces": ["error", { "skipBlankLines": false }],
     "no-undef": "error",
     "no-use-before-define": "off",
     "no-var": "error",