title: Require another file in gulpfile (which isn't in node_modules)
body: <p>I've been using gulp for a while now and know how to import another node module, e.g.</p>
  
  <pre><code>var sass = require('gulp-sass');
  </code></pre>
  
  <p>That's fine, but my gulpfile is filling up with code that I'd like to move into a separate file and "require". Specifically I am writing a postcss plugin, which I already have working when declared as a function inside of the gulpfile. My question is how to put my function in an external file and require it like I do a node module. Do I need to "export" the function in the file being required? Do I need to use ES6 modules or something like that?</p>
  
  <p>As an aside, I realise that if i was doing this probably I would either (A) turn this into a proper node module and put it on a private NPM repository, but that seems unnecessary, or (B) turn it into a proper gulp plugin, but that would require learning how to author a gulp plugin and learning about streams and stuff. Both of these are probably better but would take more time so I've decided to just keep the function simple and local for now. </p>