Turn no-tabs into a validatable rule

Closes #55.

README.md

@@ -401,31 +401,13 @@ Example ESLint configuration:
 
 ### [no-tabs]
 
-**This rule requires certain Prettier options.**
-
-This rule disallows the use of tab characters at all. It can be used just fine with Prettier as long as you don’t configure Prettier to indent using tabs.
-
-Example ESLint configuration:
-
-<!-- prettier-ignore -->
-```json
-{
-  "rules": {
-    "no-tabs": "error"
-  }
-}
-```
+**This rule requires certain options.**
 
-Example Prettier configuration (this is the default, so adding this is not required):
+This rule disallows the use of tab characters. By default the rule forbids _all_ tab characters. That can be used just fine with Prettier as long as you don’t configure Prettier to indent using tabs.
 
-<!-- prettier-ignore -->
-```json
-{
-  "useTabs": false
-}
-```
+Luckily, it’s possible to configure the rule so that it works regardless of whether Prettier uses spaces or tabs: Set `allowIndentationTabs` to `true`. This way Prettier takes care of your indentation, while the `no-tabs` takes care of potential tab characters anywhere else in your code.
 
-**Note:** Since [ESlint 5.7.0] this rule can be configured to work regardless of your Prettier configuration:
+Example ESLint configuration:
 
 <!-- prettier-ignore -->
 ```json
@@ -436,8 +418,6 @@ Example Prettier configuration (this is the default, so adding this is not requi
 }
 ```
 
-A future version of eslint-config-prettier might check for that automatically.
-
 ### [no-unexpected-multiline]
 
 **This rule requires special attention when writing code.**

bin/validators.js

@@ -39,6 +39,15 @@ module.exports = {
     return firstOption ? firstOption.allowParens === false : false;
   },
 
+  "no-tabs"(options) {
+    if (options.length === 0) {
+      return false;
+    }
+
+    const firstOption = options[0];
+    return Boolean(firstOption && firstOption.allowIndentationTabs);
+  },
+
   "vue/html-self-closing"(options) {
     if (options.length === 0) {
       return false;

test/cli.test.js

@@ -146,6 +146,7 @@ https://github.com/prettier/eslint-config-prettier#special-rules
 - curly
 - lines-around-comment
 - no-confusing-arrow
+- no-tabs
 - vue/html-self-closing
 
 The following rules are enabled but cannot be automatically checked. See:
@@ -154,7 +155,6 @@ https://github.com/prettier/eslint-config-prettier#special-rules
 - arrow-body-style
 - max-len
 - no-mixed-operators
-- no-tabs
 - no-unexpected-multiline
 - prefer-arrow-callback
 - quotes",

test/validators.test.js

@@ -60,6 +60,11 @@ rule("no-confusing-arrow", {
   invalid: [[], [null], [{ allowParens: true }], [{ other: true }]],
 });
 
+rule("no-tabs", {
+  valid: [[{ allowIndentationTabs: true }]],
+  invalid: [[], [null], [{ allowIndentationTabs: false }], [{ other: true }]],
+});
+
 rule("vue/html-self-closing", {
   valid: [
     [{ html: { void: "any" } }],