WIP: Add html-indent rule.

docs/rules/html-indent.md

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+# Enforce consistent indentation in html template (html-indent)
+
+Please describe the origin of the rule here.
+
+## :book: Rule Details
+
+This rule aims to...
+
+Examples of **incorrect** code for this rule:
+
+```html
+<template>
+    <div>
+    </div>
+</template>
+```
+
+Examples of **correct** code for this rule:
+
+```html
+<template>
+    <div>
+    </div>
+</template>
+```
+
+## :wrench: Options
+
+This rule has a mixed option:
+
+For example, for 2-space indentation:
+
+```
+vue/html-indent: [2, 2]
+```
+
+Or for tabbed indentation:
+
+```
+vue/html-indent: [2, 'tab']
+```

lib/rules/html-indent.js

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+/**
+ * @fileoverview Enforce consistent indentation in html template
+ * @author Armano
+ */
+'use strict'
+
+// ------------------------------------------------------------------------------
+// Rule Definition
+// ------------------------------------------------------------------------------
+
+function create (context) {
+  // variables should be defined here
+
+  // ----------------------------------------------------------------------
+  // Helpers
+  // ----------------------------------------------------------------------
+
+  // any helper functions should go here or else delete this section
+
+  // ----------------------------------------------------------------------
+  // Public
+  // ----------------------------------------------------------------------
+
+  return {
+    // give me methods
+  }
+}
+
+module.exports = {
+  meta: {
+    docs: {
+      description: 'Enforce consistent indentation in html template',
+      category: 'Stylistic Issues',
+      recommended: false
+    },
+    fixable: null,  // or "code" or "whitespace"
+    schema: [
+      {
+        oneOf: [
+          {
+            enum: ['tab']
+          },
+          {
+            type: 'integer',
+            minimum: 0
+          }
+        ]
+      }
+    ]
+  },
+
+  create
+}

tests/lib/rules/html-indent.js

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+/**
+ * @fileoverview Enforce consistent indentation in html template
+ * @author Armano
+ */
+'use strict'
+
+// ------------------------------------------------------------------------------
+// Requirements
+// ------------------------------------------------------------------------------
+
+const rule = require('../../../lib/rules/html-indent')
+const RuleTester = require('eslint').RuleTester
+
+// ------------------------------------------------------------------------------
+// Tests
+// ------------------------------------------------------------------------------
+
+const ruleTester = new RuleTester()
+ruleTester.run('html-indent', rule, {
+
+  valid: [
+    // give me some code that won't trigger a warning
+  ],
+
+  invalid: [
+    {
+      code: '',
+      errors: [{
+        message: 'Fill me in.',
+        type: 'Me too'
+      }]
+    }
+  ]
+})