title: Merge all columns into one without keeping colnames
body: <p>I have this format of dataframe:</p>
  
  <pre><code>df &lt;- structure(list(col1 = structure(1:2, .Label = c("text1", "text3"
  ), class = "factor"), col2 = structure(c(2L, 1L), .Label = c("text1", 
  "text2"), class = "factor"), col3 = structure(c(1L, 1L), .Label = "text1", class = "factor"), 
      col4 = structure(1:2, .Label = c("text2", "text4"), class = "factor")), .Names = c("col1", 
  "col2", "col3", "col4"), class = "data.frame", row.names = c(NA, 
  -2L))
  </code></pre>
  
  <p>And I would like to merge/combine all columns into one without keeping the column names.</p>
  
  <p>Example output:</p>
  
  <pre><code>text1
  text2
  text1
  text2
  text3
  text1
  text1
  text4
  </code></pre>
  
  <p>I found <a href="https://stackoverflow.com/questions/30277904/how-to-merge-multiple-columns-values-into-one-column">this</a> example but it merges the rows.</p>