Add an ssh_arguments configuration file field (#211)

Can be used to add SSH command-line arguments like `-o
ConnectTimeout=2`

src/config.rs

@@ -87,6 +87,7 @@ pub struct ConfigFile {
     git_repos: Option<Vec<String>>,
     disable: Option<Vec<Step>>,
     remote_topgrades: Option<Vec<String>>,
+    ssh_arguments: Option<String>,
 }
 
 impl ConfigFile {
@@ -270,6 +271,11 @@ impl Config {
         &self.config_file.remote_topgrades
     }
 
+    /// Extra SSH arguments
+    pub fn ssh_arguments(&self) -> &Option<String> {
+        &self.config_file.ssh_arguments
+    }
+
     /// Prompt for a key before exiting
     pub fn keep_at_end(&self) -> bool {
         self.opt.keep_at_end || env::var("TOPGRADE_KEEP_END").is_ok()