feat: add config to downgrade command

it's new flag --all that rollbacks all applied migrations
from database.

migra-cli/src/commands/downgrade.rs

@@ -15,7 +15,11 @@ pub(crate) fn rollback_applied_migrations(
     let applied_migrations = manager.applied_migration_names()?;
     let migrations = config.migrations()?;
 
-    let rollback_migrations_number = cmp::min(opts.migrations_number, applied_migrations.len());
+    let rollback_migrations_number = if opts.all_migrations {
+        applied_migrations.len()
+    } else {
+        cmp::min(opts.migrations_number, applied_migrations.len())
+    };
 
     for migration_name in &applied_migrations[..rollback_migrations_number] {
         if let Some(migration) = migrations.iter().find(|m| m.name() == migration_name) {

migra-cli/src/opts.rs

@@ -49,6 +49,10 @@ pub(crate) struct DowngradeCommandOpt {
     /// How many applied migrations do we have to rollback
     #[structopt(long = "number", short = "n", default_value = "1")]
     pub migrations_number: usize,
+
+    /// Rolls back all applied migrations. Ignores --number option.
+    #[structopt(long = "all")]
+    pub all_migrations: bool,
 }
 
 #[derive(Debug, StructOpt)]

migra-cli/tests/commands.rs

@@ -229,8 +229,7 @@ Pending migrations:
             .arg("-c")
             .arg(path_to_file("Migra_env.toml"))
             .arg("down")
-            .arg("-n")
-            .arg("2")
+            .arg("--all")
             .assert()
             .success();