feat: (flatten, unflatten) Add the transformKey opt.

.gitignore

@@ -1 +1,2 @@
 node_modules
+.idea

README.md

@@ -159,6 +159,55 @@ flatten({
 // }
 ```
 
+### transformKey
+
+Transform each part of a flat key before and after flattening.
+
+```javascript
+var flatten = require('flat')
+var unflatten = require('flat').unflatten
+
+flatten({
+    key1: {
+        keyA: 'valueI'
+    },
+    key2: {
+        keyB: 'valueII'
+    },
+    key3: { a: { b: { c: 2 } } }
+}, {
+    transformKey: function(key){
+      return '__' + key + '__';
+    }
+})
+
+// {
+//   '__key1__.__keyA__': 'valueI',
+//   '__key2__.__keyB__': 'valueII',
+//   '__key3__.__a__.__b__.__c__': 2
+// }
+
+unflatten({
+      '__key1__.__keyA__': 'valueI',
+      '__key2__.__keyB__': 'valueII',
+      '__key3__.__a__.__b__.__c__': 2
+}, {
+    transformKey: function(key){
+      return key.substring(2, key.length - 2)
+    }
+})
+
+// {
+//     key1: {
+//         keyA: 'valueI'
+//     },
+//     key2: {
+//         keyB: 'valueII'
+//     },
+//     key3: { a: { b: { c: 2 } } }
+// }
+```
+
 ## Command Line Usage
 
 `flat` is also available as a command line tool. You can run it with 

index.js

@@ -4,11 +4,16 @@ module.exports = flatten
 flatten.flatten = flatten
 flatten.unflatten = unflatten
 
+function keyIdentity (key) {
+  return key
+}
+
 function flatten (target, opts) {
   opts = opts || {}
 
   var delimiter = opts.delimiter || '.'
   var maxDepth = opts.maxDepth
+  var transformKey = opts.transformKey || keyIdentity
   var output = {}
 
   function step (object, prev, currentDepth) {
@@ -24,8 +29,8 @@ function flatten (target, opts) {
       )
 
       var newKey = prev
-        ? prev + delimiter + key
-        : key
+        ? prev + delimiter + transformKey(key)
+        : transformKey(key)
 
       if (!isarray && !isbuffer && isobject && Object.keys(value).length &&
         (!opts.maxDepth || currentDepth < maxDepth)) {
@@ -46,6 +51,7 @@ function unflatten (target, opts) {
 
   var delimiter = opts.delimiter || '.'
   var overwrite = opts.overwrite || false
+  var transformKey = opts.transformKey || keyIdentity
   var result = {}
 
   var isbuffer = isBuffer(target)
@@ -71,7 +77,7 @@ function unflatten (target, opts) {
   })
 
   sortedKeys.forEach(function (key) {
-    var split = key.split(delimiter)
+    var split = key.split(delimiter).map(transformKey)
     var key1 = getkey(split.shift())
     var key2 = getkey(split[0])
     var recipient = result