Update: Add never option to arrow-body-style (fixes eslint#6317)

Arrow functions that return object literals can look very similar to arrow functions with brace bodies.  Some syntactic ambiguity can be avoided by disallowing block-style arrow functions in favour of ES5 function expressions.

**Outcome**

The following patterns are considered problems:
```
/*eslint arrow-body-style: ["error", "never"]*/
/*eslint-env es6*/

let foo = () => {
    return 0;
};

let foo = (retv, name) => {
    retv[name] = true;
    return retv;
};
```

The following patterns are not considered problems:
```
/*eslint arrow-body-style: ["error", "never"]*/
/*eslint-env es6*/

let foo = () => 0;

let foo = () => ({ key: 0 });
```

lib/rules/arrow-body-style.js

@@ -18,14 +18,15 @@ module.exports = {
 
         schema: [
             {
-                enum: ["always", "as-needed"]
+                enum: ["always", "as-needed", "never"]
             }
         ]
     },
 
     create: function(context) {
         var always = context.options[0] === "always";
         var asNeeded = !context.options[0] || context.options[0] === "as-needed";
+        var never = context.options[0] === "never";
 
         /**
          * Determines whether a arrow function body needs braces
@@ -36,18 +37,26 @@ module.exports = {
             var arrowBody = node.body;
 
             if (arrowBody.type === "BlockStatement") {
-                var blockBody = arrowBody.body;
-
-                if (blockBody.length !== 1) {
-                    return;
-                }
-
-                if (asNeeded && blockBody[0].type === "ReturnStatement") {
+                if (never) {
                     context.report({
                         node: node,
                         loc: arrowBody.loc.start,
                         message: "Unexpected block statement surrounding arrow body."
                     });
+                } else {
+                    var blockBody = arrowBody.body;
+
+                    if (blockBody.length !== 1) {
+                        return;
+                    }
+
+                    if (asNeeded && blockBody[0].type === "ReturnStatement") {
+                        context.report({
+                            node: node,
+                            loc: arrowBody.loc.start,
+                            message: "Unexpected block statement surrounding arrow body."
+                        });
+                    }
                 }
             } else {
                 if (always) {