phantom = function( markup, fOut ) {

  // Save the markup to a temporary file
  const tempFile = fOut.replace(/\.png$/i, '.png.html');
  FS.writeFileSync(tempFile, markup, 'utf8');
  const scriptPath = SLASH( PATH.relative( process.cwd(),
    PATH.resolve( __dirname, '../utils/rasterize.js' ) ) );
  const sourcePath = SLASH( PATH.relative( process.cwd(), tempFile) );
  const destPath = SLASH( PATH.relative( process.cwd(), fOut) );
  SPAWN('phantomjs', [ scriptPath, sourcePath, destPath ]);
}
Generate a PDF from HTML using Phantom's CLI interface.
Spawns a child process with `phantomjs <script> <source> <target>`. Phantom
must be installed and path-accessible.
TODO: If HTML generation has run, reuse that output
TODO: Local web server to ease Phantom rendering