Add support for tags: if_blank and if_not_blank

[adamjohnson]

If you open the backpage of the Design System in hammer right now, it looks like this:

How to reproduce

1. Start hammer: hammer-start
2. Navigate to the backpage template in the design system: http://localhost:2000/wvu-design-system/views/backpage.html

Tags to add support for

if_blank:

<r:if_blank value="{$variableDoesNotExist}">
  <h2><code>if_blank</code> works!</h2>
</r:if_blank>

if_not_blank:

<r:if_not_blank value="{$nav_dropdowns}">
  <h2><code>if_not_blank</code> works!</h2>  
</r:if_not_blank>

Link to tags in CleanSlate repo

cleanslate/app/lib/slate/radius/tags/basic.rb (Github)

These tags are one of three sets of tags heavily used in Design System components.