Add something to Conditionals episode to cover 'truthiness' of things in MATLAB

[cjfullerton]

The True & False challenge has been removed. The one useful discussion that come from that was the idea that everything in MATLAB is considered either true or false.

A callout section could be added to re-introduce this idea.

[fherreazcue]

should this be part of the section at the beginning where we cover logical operations? (&& and ||)