Add responsive and visibility helper classes

[ulinaaron]

Currently the distribution file, tent.min.css weighs in at 3.7kb gzipped. I am fine with the file growing in size as long as it stays under 5kb gzipped and the inclusions make sense for the overall goals of the framework. The framework will not grow much in size to allow for "pretty" components anyways. So I believe it is advantageous to include many more responsive and visibility classes based around the common breakpoints.

So far I am thinking of general show and hide classes based around the media query breakpoints we currently have defined. These classes could be applied to any element that you'd like to have act different on a certain breakpoint or set of breakpoints.

We already have responsive utilities for flex ordering, so we're set there. What may need to happen is improved documentation that .flex-first--#BP can be used in combination with the grid rather than created repeated classes such as .grid-first--#BP

Any other responsive utility classes feedback is welcome!