Improve decimal part formatting

[taflanidi]

Prerequisites

• [X] Put an X between the brackets on this line if you have done all of the following:
  • read our Wiki;
  • read the entire Known issues section;
  • checked that my feature request isn't already filled;
  • searched StackOverflow's input-mask tag for similar problems.

Summary

Make sure right-to-left masks work fine with optional decimals.

Also, add a Wiki page with a tutorial for this feature.