Added support for Access Tokens and improved message conditions

[TheNaubit]

First of all, thanks for building this, this was a really useful starting tool for integrating Ntfy into my system.

I needed a few changes and I wanted to share them in case it is useful for others. I added the following:

• Support for access tokens. In case you have a secured Ntfy server, you might need to use it. You can now set the env var called NTFY_ACCESS_CONTROL with an access token you created (without the Bearer string), as explained here: https://docs.ntfy.sh/publish/#access-tokens If you are not in need of access tokens, you can just leave this var unset.
• I had an issue where I needed to send a message without a title. So I modified the conditions so it ignores the message if there is no title and no message or both are empty strings. I also made it so that if there is a title, we send the title section of the message in bold, using MarkdownV2.