title: Create a custom task to cache data to html on demand
body: <p>I have an external resource (API) that I want to hit and generate some html pages. The procedure is easy to do (fetch data, generate html, save to file). The problem that I am facing is how to script that in order to invoke the procedure on demand. 
  I know about pre/postbuild scripts but I don't want to run this thing on every publish</p>
  
  <p>Being a Django dev I was able extend the command line tooling and add custom commands to do this sort of things. </p>
  
  <p>Is there any way to do this? I use gulp. Should I add this functionality there?</p>