Add smtp client support for sending mail in UI

`mailer_command` was removed, and a new setting `send_mail` was added.

Its possible values are a string, consisting of a shell command to
execute, or settings to configure an smtp server connection. The
configuration I used for testing this is:

  [composing]
  send_mail = { hostname = "smtp.mail.tld", port = 587, auth = { type = "auto", username = "yoshi", password = { type = "command_eval", value = "gpg2 --no-tty -q -d ~/.passwords/msmtp/yoshi.gpg" } }, security = { type = "STARTTLS" } }

For local smtp server:
  [composing]
  send_mail = { hostname = "localhost", port = 25, auth = { type = "none" }, security = { type = "none" } }

1 files changed, 2 insertions, 1 deletions

diff --git a/Cargo.toml b/Cargo.toml
index 43a62165..0da12aab 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -69,10 +69,11 @@ debug = false
 members = ["melib", "testing", ]
 
 [features]
-default = ["sqlite3", "notmuch", "regexp"]
+default = ["sqlite3", "notmuch", "regexp", "smtp"]
 notmuch = ["melib/notmuch_backend", ]
 jmap = ["melib/jmap_backend",]
 sqlite3 = ["melib/sqlite3"]
+smtp = ["melib/smtp"]
 regexp = ["pcre2"]
 cli-docs = []
 svgscreenshot = ["svg_crate"]