Add sudo_command option (#379)

This allows the user to specify the preferred sudo command to be used
instead of the command chosen by Sudo::detect

config.example.toml

@@ -13,6 +13,9 @@
 # Do not ask to retry failed steps (default: false)
 #no_retry = true
 
+# Sudo command to be used
+#sudo_command = "sudo"
+
 # Run `sudo -v` to cache credentials at the start of the run; this avoids a
 # blocking password prompt in the middle of a possibly-unattended run.
 #pre_sudo = false