title: finding combinations of elements from two matrices
body: <p>I would like a matrix of all the combinations of</p>
  <pre><code>matrix1 &lt;- combinations(10,3)
  matrix2 &lt;- combinations(20,1)
  </code></pre>
  <p>i.e. we are choosing 3 numbers without repetition from a bag with numbers 1 to 10, then one number from 1 to 20</p>