title: join/merge two columns inside a data-frame
body: <p>Say I have this data,</p>
  
  <pre><code>(df &lt;- data.frame( col1 = c('My','Your','His','Thir'), col2 = c('Cat','Dog','Fish','Dog')))
  
    col1 col2
  1   My  Cat
  2 Your  Dog
  3  His Fish
  4 Thir  Dog
  </code></pre>
  
  <p>and I want to combine the columns like this</p>
  
  <pre><code>`some magic`
  
    col1 col2      col3
  1   My  Cat    My Cat
  2 Your  Dog  Your Dog
  3  His Fish  His Fish
  4 Thir  Dog  Thir Dog
  </code></pre>
  
  <p>What do I do? maybe even with a comma (,) like this,</p>
  
  <pre><code>`some magic`
  
    col1 col2      col3
  1   My  Cat    My, Cat
  2 Your  Dog  Your, Dog
  3  His Fish  His, Fish
  4 Thir  Dog  Thir, Dog
  </code></pre>