Add support for extra sendmail arguments (#2731)

* Add support for extra sendmail arguments

* Sendmail args to exec.command should be a list

* Add go-shellquote package

* Use go-shellquote lib for parsing Sendmail args

* Only parse if sendmail is configured

vendor/github.com/kballard/go-shellquote/doc.go

@@ -0,0 +1,3 @@
+// Shellquote provides utilities for joining/splitting strings using sh's
+// word-splitting rules.
+package shellquote