[#13] implement html filter
Created by: pbatroff
I copied the class and functionality from flexmailer and added it to _civicrm_api3_job_mosaico_msg_filter. I tested this on CiviCRM 5.13.4 with mosaico 2.2, but I don't se it not working in newer versions of Civi/Mosaico.
The code is straight forward and the same as flexmailer.
@MikeyMJCO @artfulrobot Could one of you review this and then merge the PR please?