* Masking Character added, not yet used.
* Setting the masking character can be chained with other extensions.
* Added string extension for masking, and replaced hardcoded asterisks.
* Check if mask is null first.
* Fixed Typo in previous test and added new test for custom masks.
* Added tests for masking with null character
* Added docs and example.
* Adjusted extensions so that Mask is integrated into Secret extension. Updated Exampls and Tests accordingly