Added option to only notify about important updates

Added option to only notify about major updates. This setting is now the default and can be changed in the config by changing the setting "NOTIFY_UPDATES" from "major" to "all".

This setting was achieves by turning the previous if statement into an if-else statement with the new option. For this, I utilized a function that gets the latest tag from the GitHub repository.

I wasn't able to implement the 'if URL exists' check, the URL would just not return an error negating the function. I will probably fix this in the future, but as it is now, if the GitHub API server can't be reached this might trow an error.

The major release is still the previous update version retreated from the GitHub repository. This means I will only update that version for major or otherwise important updates. 

I implemented this feature because I didn't want to spam new users with a new update notification every other day.