Masked or Safe CharSequence

[dmfs]

In order to replace characters in a CharSequence which are invalid for certain applications with a specific "default" character (as opposed to removing them) I want a Masked decorator for CharSequences.

The name could be Masked, Safe or anything else that expresses that certain chars have been replaced with a default. The Characters to retain (or to replace) could be given by a CharSet as in #82.