Fetch URL from Heroku environment variable

[bmann]

This is the line where the URL of the script is set. It is currently hard coded.

https://github.com/bmann/rss2html/blob/76a45c639cc1baa4c85acc7bead31209a8a394e6/scripts/parse.js#L24

Right now, the work around is that you just edit this file and the URL of your own server.

To fix it, need a way to get the APP_URL environment variable from your Heroku app.