title: Environment variable in YAML file for Ceedling not evaluated
body: <p>I have a Ceedling project for which i want to setup ceedling paths which are not hardcoded.
  For this I tried both ways - </p>
  
  <pre><code>:environment:
    - :CEEDLING: 
       - my_ceed/ceedling
  </code></pre>
  
  <p>and</p>
  
  <pre><code>:environment:
    - :CEEDLING: 
       - "#{ENV['CEEDLING_PATH']}"
  </code></pre>
  
  <p>and using it like </p>
  
  <pre><code>:which_ceedling: ../../../#{ENV['CEEDLING']}
  </code></pre>
  
  <p>However this is not evaluated as expected -</p>
  
  <pre><code>Traceback (most recent call last):
      3: from /usr/local/bin/ceedling:23:in `&lt;main&gt;'
      2: from /usr/local/bin/ceedling:23:in `load'
      1: from /var/lib/gems/2.5.0/gems/ceedling-0.29.1/bin/ceedling:316:in `&lt;top (required)&gt;'
  /var/lib/gems/2.5.0/gems/ceedling-0.29.1/bin/ceedling:316:in `load': cannot load such file -- #{ENV['CEEDLING']}/lib/ceedling.rb (LoadError)
  </code></pre>
  
  <p>What am i missing here or is it not possible to do this for ceedling paths itself?</p>