v3.0.0

This is a major update to the sink, switching to MailKit on all platforms, and with new configuration methods including JSON configuration syntax.

• #87 - support MailKit's SecureSocketOptions (@arthurchiakh)
• #98 - extract email transport from sink (@mgrosperrin)
• #97 - support batched email with headers and footers (@0xced)
• #122, #123 - use MailKit on all platforms, redesign configuration methods, drop obsolete TFMs (@nblumhardt)

Breaking changes

• Drops support for netstandard1.x and net45
• Now relies on MailKit for email delivery on all target platforms
• Redesigned WriteTo.Email() configuration method and parameters mean code and JSON configuration will need to be updated, see README.md for details