Add a "display unprintable chars" toggle mode

As mentioned in #1268 ... because I'm a purist and because I work with UTF-8 quite a bit and unprintable control characters are significant to my work...

I want to see in OpenRefine a toggle mode for DISPLAY UNPRINTABLE CHARS for our users (like the ACK and CR/LF below)...so that they show up and are highlighted with special coloring/font. This toggle mode should work perhaps on both datagrids & facets, and perhaps other dialogs or extension mechanisms, yet to be determined.

This is work to be done in the future in part of our UI Enhancement project(s).

We will want to utilize CSS styling for this and not actual Javascript replacement characters.
An example of how CSS Styling in a cell could be utilized for the various unprintable control characters:
http://mleibman.github.io/SlickGrid/examples/example8-alternative-display.html

Another alternative might be https://developer.mozilla.org/en-US/docs/Web/SVG/Element/symbol and calling <use> whenever we need to show unprintable characters

Start with the UI Enhancement project and review the datagrids and facets alongside the linked SlickGrid alternative-display example. Define which interfaces the toggle supports and how control characters are highlighted with CSS; done means unprintable characters are visible without JavaScript replacement characters.