adding custom string replacer for flex replace

[nhan2804]

The first, thank you for creating this library, it's very good for me and community. But when using, sometimes i want to use value of rest stringReplace to create another stringReplace , so here is my pull request details.

1. add callback function result into object of array stringReplacers for custom dynamic valueSlot
2. add callback fuction newSlot into object of array dynamicReplacers for get and custom rest replacer
   Please check pull request and accept it