title: Append line after last match with sed
body: <p>Let's assume I have the following input.</p>
  
  <pre><code>Header
  thing0 some info
  thing4 some info
  thing4 some info
  thing4 some info
  thing2 some info
  thing2 some info
  thing3 some info
  </code></pre>
  
  <p>Now, I want to be able to append "foo" on the last successful match of "thing4" like this.</p>
  
  <pre><code>Header
  thing0 some info
  thing4 some info
  thing4 some info
  thing4 some info
  foo
  thing2 some info
  thing2 some info
  thing3 some info
  </code></pre>
  
  <p>The order is not necessarily  guaranteed, but the sequential numbering in this example is just to show that there is a searchable keyword before certain lines of text and that they are are <strong>usually</strong> grouped together on input, but it is not guaranteed.</p>