feat(functionValue): format functions output the way you want

`functionValue` can now be used to configure the formatting of functions

README.md

@@ -86,6 +86,13 @@ console.log(reactElementToJSXString(<div a="1" b="2">Hello, world!</div>));
 
   If false, functions bodies are replaced with `function noRefCheck() {}`.
 
+**options.functionValue: function, default `(fn) => fn`**
+
+  Allows you to override the default formatting of function values.
+
+  `functionValue` receives the original function reference as input
+  and should send any value as output.
+
 **options.tabStop: number, default 2**
 
   Provide a different number of columns for indentation.

index-test.js

@@ -748,4 +748,19 @@ describe('reactElementToJSXString(ReactElement)', () => {
   </div>
 </div>`);
   });
+  it('should return functionValue result when it returns a string', () => {
+    expect(
+      reactElementToJSXString(<div fn={() => 'value'}/>, {showFunctions: true, functionValue: () => '...'})
+  ).toEqual('<div fn={...} />');
+  });
+  it('sends the original fn to functionValue', () => {
+    const fn = () => {};
+    const functionValue = receivedFn => expect(receivedFn).toBe(fn);
+    reactElementToJSXString(<div fn={fn} />, {functionValue});
+  });
+  it('should return noRefCheck when "showFunctions" is false and "functionValue" is not provided', () => {
+    expect(
+      reactElementToJSXString(<div fn={() => {}}/>)
+    ).toEqual('<div fn={function noRefCheck() {}} />');
+  });
 });

index.js

@@ -7,12 +7,15 @@ import sortobject from 'sortobject';
 import traverse from 'traverse';
 import {fill} from 'lodash';
 
+const defaultFunctionValue = fn => fn;
+
 export default function reactElementToJSXString(
   ReactElement, {
     displayName,
     filterProps = [],
     showDefaultProps = true,
     showFunctions = false,
+    functionValue = defaultFunctionValue,
     tabStop = 2,
     useBooleanShorthandSyntax = true,
     maxInlineAttributesLineLength,
@@ -190,12 +193,21 @@ got \`${typeof Element}\``
     return typeof value;
   }
 
+  function isFunction (value) {
+    return typeof value === 'function';
+  }
+
   function formatValue(value, inline, lvl) {
     const wrapper = '__reactElementToJSXString__Wrapper__';
+    if (isFunction(value)) {
+      return functionValue(
+        showFunctions === false && functionValue === defaultFunctionValue ?
+          function noRefCheck() {} : // eslint-disable-line prefer-arrow-callback
+          value
+      );
+    }
 
-    if (typeof value === 'function' && !showFunctions) {
-      return function noRefCheck() {};
-    } else if (isElement(value)) {
+    if (isElement(value)) {
       // we use this delimiter hack in cases where the react element is a property
       // of an object from a root prop
       // i.e.