Release v2.3.1

Fixed:
- go-mail#39: Reverts addition of Go modules `go.mod` manifest.

Release v2.3.0

Added:
- go-mail#12: Adds `SendError` to provide additional info about the cause and index of
  a failed attempt to transmit a batch of messages.
- go-gomail#78: Adds new `Message` methods for attaching and embedding
  `io.Reader`s: `AttachReader` and `EmbedReader`.
- go-mail#39: Adds support for Go modules (Go 1.11+).

Fixed:
- go-mail#26: Fixes RFC 1341 compliance by properly capitalizing the
  `MIME-Version` header.
- go-mail#30: Fixes IO errors being silently dropped in `Message.WriteTo`.

Release 2.2.0

Release 2.1.0

Gomail 2.0.0