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/README

@@ -0,0 +1,36 @@
+PACKAGE
+
+package shellquote
+    import "github.com/kballard/go-shellquote"
+
+    Shellquote provides utilities for joining/splitting strings using sh's
+    word-splitting rules.
+
+VARIABLES
+
+var (
+    UnterminatedSingleQuoteError = errors.New("Unterminated single-quoted string")
+    UnterminatedDoubleQuoteError = errors.New("Unterminated double-quoted string")
+    UnterminatedEscapeError      = errors.New("Unterminated backslash-escape")
+)
+
+
+FUNCTIONS
+
+func Join(args ...string) string
+    Join quotes each argument and joins them with a space. If passed to
+    /bin/sh, the resulting string will be split back into the original
+    arguments.
+
+func Split(input string) (words []string, err error)
+    Split splits a string according to /bin/sh's word-splitting rules. It
+    supports backslash-escapes, single-quotes, and double-quotes. Notably it
+    does not support the $'' style of quoting. It also doesn't attempt to
+    perform any other sort of expansion, including brace expansion, shell
+    expansion, or pathname expansion.
+
+    If the given input has an unterminated quoted string or ends in a
+    backslash-escape, one of UnterminatedSingleQuoteError,
+    UnterminatedDoubleQuoteError, or UnterminatedEscapeError is returned.
+
+