title: Check whether string is present in multiple columns and return true/false column
body: <p>Dataset similar to:</p>
  
  <pre><code>Day   Age    Med1, Med2, Med3, Med4 ... Med 18
   M     42     En    Fu    Gt    Di       Mo
   F     36     Gt    Pa    Mo    En       Mo
   F     89     Gt    Pa    Pa    En       Na      
  </code></pre>
  
  <p>I want to add a column returning True/False if, for example, 'Mo' is present in the Med1 - Med18 columns. </p>
  
  <pre><code>Day   Age    Med1, Med2, Med3, Med4 ... Med 18  MoGiven
   M     42     En    Fu    Gt    Di       Mo        T
   F     36     Gt    Pa    Mo    En       Mo        T
   F     89     Gt    Pa    Pa    En       Na        F
  </code></pre>
  
  <p>Have looked at various other threads and closest solution seems to be:</p>
  
  <pre><code>raw &lt;- imported_data %&gt;%
    mutate_at(var(contains("Med")), funs(.=='Mo')
  </code></pre>
  
  <p>but am getting error <code>no applicable method for 'tbl_vars' applied to an object of class "quosures"</code></p>
  
  <p>Would like to avoid doing a nested ifelse for each column if possible, unless this is only way.</p>
  
  <p>Am I on the right lines?</p>