[FEATURE] Handle index.php cli calls as web requests.

- Receive an URL and dispatch the call as if it was called
  from a web browser
- Option --method allows to specify which HTTP method to simulate,
  since some methods by check $_POST or $_GET specifically instead
  of $_REQUEST
- Show an error message if index.php is called with invalid arguments
  from the cli.
[FEATURE] Cache the Sparkle appcast.xml feed

- appcast.xml files are written to releases/<tool>/appcast.xml
  if it doesn't exist, so subsequent calls of /releases/<tool>/appcast.xml
  deliver the itself, and don't have to go through php.