Read/write from/to .VCF file

[vestrel00]

Enables import existing contacts from and export to .VCF file.

Additionally, new (raw) contacts (not in the database) or existing (raw) contacts with unsaved changes will be exportable. Consumers will be able to specify fields to export instead of the entire contact!

If this will require external dependencies, it should be in a separate module.

• Add docs
• Demonstrate functionality in sample app

[vestrel00]

As discussed in #323 (comment), I should probably use https://github.com/mangstadt/ez-vcard to implement this!