Add support for Value property on MultiLineTextAttributes

Added Value property to MultiLineTextAttribute so that someone can create their own help text.

Also refactored AddToHelpText() to allow for externally calling formatting engine of CommandLineParser so that, in the future, can easily allow someone to leverage the formatting capabilities of CommandLineParser/MultiLineTextAttributes.  In other words, future support of a function called GetFormattedValue on MultiLineTextAttribute).

Also added tests for new Value property in a new fixture.  Hopefully put the new fixture in the right place ;-).