Allow more than one custom notification

[Lineflyer]

As the custom notification system allows conditions to be set, it would be beneficial if more than one custom noctification at the same time are supported.

Example:

• Notify kind == "nightly" about a new feature
• In parallel notify versionName == "20160101" that this version does no longer work

To work smoothly this would also require to selectively clear single notifications of course.

Kind of depends on #17 to work as intendend