Add support for tags: if_true and if_false

[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_true:

<r:if_true value="{$myVar}">
  <h2><code>$myVar</code> is true</h2>
</r:if_true>

if_false:

<r:if_false value="{$myVar}">
  <h2><code>$myVar</code> is false</h2>
</r:if_false>

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.