Add eslint with CI staged files linting

Add lint:ci script

This will lint all changed files in the branch / Travis commit range.

Update Travis config to lint changed files before build

.eslintrc.js

@@ -0,0 +1,61 @@
+module.exports = {
+  extends: 'standard',
+  globals: {
+    $: false
+  },
+  rules: {
+    'semi': [2, 'always'],
+    'no-mixed-operators': 'off',
+    'no-unused-expressions': 'off',
+    'no-var': 'error',
+    'one-var': [2, 'never'],
+    'prefer-const': 'error',
+    'operator-linebreak': [
+      'error',
+      'after',
+      { overrides: { '?': 'before', ':': 'before', '|>': 'before' } }
+    ],
+    radix: 'error',
+    'require-await': 'error'
+  },
+  env: {
+    browser: true
+  },
+  overrides: [
+    {
+      files: ['src/scripts/*.js'],
+      globals: {
+        db: false,
+        togglbutton: false,
+        TogglButton: false,
+        chrome: false,
+        ga: false
+      }
+    },
+    {
+      files: ['src/scripts/content.js'],
+      globals: {
+        createTag: false
+      }
+    },
+    {
+      files: ['src/scripts/common.js'],
+      globals: {
+        createTag: false
+      }
+    },
+    {
+      files: ['src/scripts/popup.js'],
+      globals: {
+        PopUp: false,
+        Db: false
+      }
+    },
+    {
+      files: ['src/scripts/content/*.js'],
+      globals: {
+        togglbutton: false
+      }
+    }
+  ]
+};

.gitignore

@@ -2,6 +2,7 @@
 !.editorconfig
 !.gitignore
 !.travis.yml
+!.eslintrc.js
 
 dist/
 node_modules/

.travis.yml

@@ -3,7 +3,7 @@ node_js:
   - 8.11.3
 
 before_script:
-  - npm run xo
+  - npm run lint:ci
 
 script:
   - npm run build