Add support for tag: if_then_else

[adamjohnson]

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

if_then_else is used by the Masthead.

Tag to add support for

if_then_else:

<h2>
  <r:if_then_else test="{{ NOT(BLANK(custom_page_name)) }}" then="{$custom_page_name}" else="{page:name}" />
</h2>

Link to tags in CleanSlate repo

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