feat: add command to upgrade command

... which adds the possibility to specify the number of
existing migrations to be updated

migra-cli/src/commands/upgrade.rs

@@ -27,7 +27,11 @@ pub(crate) fn upgrade_pending_migrations(config: Config, opts: UpgradeCommandOpt
             }
         }
     } else {
-        for migration in pending_migrations.iter() {
+        let upgrade_migrations_number = opts
+            .migrations_number
+            .unwrap_or_else(|| pending_migrations.len());
+
+        for migration in &pending_migrations[..upgrade_migrations_number] {
             print_migration_info(migration);
             manager.upgrade(migration)?;
         }

migra-cli/src/opts.rs

@@ -51,6 +51,10 @@ pub(crate) struct UpgradeCommandOpt {
     /// in the database.
     #[structopt(long = "name")]
     pub migration_name: Option<String>,
+
+    /// How many existing migrations do we have to update.
+    #[structopt(long = "number", short = "n")]
+    pub migrations_number: Option<usize>,
 }
 
 #[derive(Debug, StructOpt)]

migra-cli/tests/commands.rs

@@ -246,13 +246,8 @@ Pending migrations:
             .arg("-c")
             .arg(path_to_file("Migra_env.toml"))
             .arg("up")
-            .assert()
-            .success();
-
-        Command::cargo_bin("migra")?
-            .arg("-c")
-            .arg(path_to_file("Migra_env.toml"))
-            .arg("down")
+            .arg("-n")
+            .arg("1")
             .assert()
             .success();