[enhancement] Add custom audio cue triggers

[Git-GT]

The current runnerup allows to configure the audio cues using time intervals or distance intervals. It would be nice if it would provide a way to configure also a custom list of triggers. For example setting a trigger at 30Km another at 2h from the beginning, another at 35Km etc (mixing distance and time triggers and without periodic intervals)

[gerhardol]

One way to do this is multiple simultaneous audio triggers. https://github.com/jonasoreland/runnerup/wiki/TODO-list