Support querying unknown executed migrations

[medranocalvo]

In order to check whether the database schema is up-to-date, the umzug.pending() method can be used.

I'm trying to check whether the application is up-to-date to the schema; that is, whether the user is running an older version of an application which is no longer compatible to the already migrated schema.

Initially I though of using umzug.executed() and compare it with umzug.migrations(). That does not work, because Umzug filters the return value of the former with the latter.

I propose a new feature be added to Umzug that allows doing the above check. I thought of two possible implementations:

1. Augment umzug.executed() to accept a new optional parameter all. When true, all executed migrations should be returned, no matter whether they exist or not.
2. Add an additional method e.g. umzug.unknown() which lists the unknown migrations. This has the disadvantage that we would not be able to distinguish in which order they were applied.

Please find below a quick patch implementing option 1.

diff --git i/README.md w/README.md
index 611062d..b90740f 100644
--- i/README.md
+++ w/README.md
@@ -169,6 +169,13 @@ const migrations = await umzug.executed()
 // returns an array of all already executed migrations
 ```
 
+The complete list of executed migrations, included those which no longer exist, can be retrieved passing `true` as a parameter:
+
+```js
+const migrations = await umzug.executed(true)
+// returns an array of all migrations registered as executed
+```
+
 #### Executing pending migrations
 
 The `up` method can be used to execute all pending migrations.
diff --git i/src/umzug.ts w/src/umzug.ts
index 4f4ecc0..9d4cb26 100644
--- i/src/umzug.ts
+++ w/src/umzug.ts
@@ -170,19 +170,23 @@ export class Umzug extends emittery {
+  async executed(all?: boolean): Promise {
     return this.runCommand('executed', async ({context}) => {
-      const list = await this._executed(context)
+      const list = await this._executed(context, all)
       // We do the following to not expose the `up` and `down` functions to the user
       return list.map(m => ({name: m.name, path: m.path}))
     })
   }
 
   /** Get the list of migrations which have already been applied */
-  private async _executed(context: Ctx): Promise>> {
+  private async _executed(context: Ctx, all?: boolean): Promise>> {
     const [migrations, executedNames] = await Promise.all([this.migrations(context), this.storage.executed({context})])
-    const executedSet = new Set(executedNames)
-    return migrations.filter(m => executedSet.has(m.name))
+    if (all) {
+      return executedNames;
+    } else {
+      const executedSet = new Set(executedNames)
+      return migrations.filter(m => executedSet.has(m.name))
+    }
   }
 
   /** Get the list of migrations which are yet to be applied */