feat: Add excludeVariableMatch option to require-variable-type

Enable ignoring variables that match a certain regex

.README/rules/require-variable-type.md

@@ -2,4 +2,25 @@
 
 Requires that all variable declarators have type annotations.
 
+#### Options
+
+You can exclude variables that match a certain regex by using `excludeVariableMatch`.
+
+This excludes all parameters that start with an underscore (`_`).
+The default pattern is `a^`, which doesn't match anything, i.e., all parameters are checked.
+
+```js
+{
+    "rules": {
+        "flowtype/require-variable-type": [
+            2,
+            {
+              "excludeVariableMatch": "^_"
+            }
+        ]
+    }
+}
+```
+
+
 <!-- assertions requireVariableType -->

src/rules/requireVariableType.js

@@ -11,13 +11,20 @@ export default (context) => {
     return () => {};
   }
 
+  const excludeVariableMatch = new RegExp(_.get(context, 'options[0].excludeVariableMatch', 'a^'));
+
   return {
     VariableDeclaration: (variableDeclaration) => {
       _.forEach(variableDeclaration.declarations, (variableDeclarator) => {
         const identifierNode = _.get(variableDeclarator, 'id');
-        const typeAnnotation = _.get(identifierNode, 'typeAnnotation');
         const identifierName = _.get(identifierNode, 'name');
 
+        if (excludeVariableMatch.test(identifierName)) {
+          return;
+        }
+
+        const typeAnnotation = _.get(identifierNode, 'typeAnnotation');
+
         if (!typeAnnotation) {
           context.report({
             data: {

tests/rules/assertions/requireVariableType.js

@@ -15,15 +15,35 @@ export default {
           message: 'Missing "bar" variable type annotation.'
         }
       ]
+    },
+    {
+      code: 'var _foo = "bar", bar = 1',
+      errors: [
+        {
+          message: 'Missing "bar" variable type annotation.'
+        }
+      ],
+      options: [
+        {
+          excludeVariableMatch: '^_'
+        }
+      ]
     }
-
   ],
   valid: [
     {
       code: 'var foo : string = "bar"'
     },
     {
       code: 'var foo : string = "bar", bar : number = 1'
+    },
+    {
+      code: 'var _foo = "bar", bar : number = 1',
+      options: [
+        {
+          excludeVariableMatch: '^_'
+        }
+      ]
     }
   ]
 };