[feature] support of caldav

[bobuk]

This is an open request for any interested developer.

Many users are asking if we can add support for non-Google calendars with CalDav protocol.
Check this Issue if you would like to pursue this idea.

[L0ndra]

@Ceesaxp i am sure that it's more)

[Menelion]

@Ceesaxp Can I be of any help? Go is not my main language, but the project is awesome and I need it for my flow, but I'd like to sync between Google and Microsoft Exchange. Where are you in this, do you need help? Thanks!