Change agenda bullet

[quite]

I personally think that the more visible * or - could be used as the bullets on the agenda lines, instead of the tiny ·. But it is indeed a matter of personal taste. What do you think? Could also be made configurable, but perhaps that would be over-the-top.

[b1rger]

I'm currently evaluating replacing the whole output code using minijinja as a templating engine. This is mostly due to #124, but this request would also be fixed by that. We would then ship default templates for printing the calendar and the agenda, but users would be able to override those templates in their configuration