Allow splitting and importing configurations

[Jojodicus]

After using and managing CalUnite for a while, the config can get quite big and hard to navigate. Having a way to split up the config into multiple parts and being able to include parts into the main config would be very appreciated.

Standard YAML does not provide such a feature unfortunately. So the config parser would have to be extended, adding the use of a template preprocessor of some kind.

When adding one, we can also reconsider #3, as there is now a rather easy way to add that functionality.

[Jojodicus]

Depending on which template engine is used, this can potentially break older configs. So bump major version.

[sdenike]

I think this would be awesome to be able to split up configs, as I had mentioned before I have a couple family members and coworkers that would love to be able to merge some calendars like you and I do, but they wouldn't know the first thing about setting up docker, etc and rather than having a monster of a config file if calunite could read from I dunno, a data/ directory and in that directory have sub directories such as data/shelby/ and data/johannes/ and inside each subfolder have a config file that contains only their calendars, or perhaps just within data have shelby.yml, johannes.yml each containin our own information.

To expand on this, if each config could specify how often it updates, and if nothing is defined, it would default to what is specified in the Docker Compose file.

The last part about the update time may be something for another ticket, but thought of mentioning it here as I do have a few calendar URLs in mine that I could get away with updating daily or longer since once those calendars have been made they rarely if ever change and requesting it every X minutes is a waste of processing power/bandwidth(I am sure the bandwidth is minimal based on the size of most .ics files but just a thought)

[Jojodicus]

Very good idea. Can you make a separate issue for that? Preferably also with a minimal example of how it would look in the config.