Add bw get notes <id> command (#255)

Rationale: the notes object is a freeform plain-text field that's
prominently displayed in Web Vault. It is also useful for the CLI users
as discussed before in issues #81 and #196. I have some use cases
planned myself.

I was rather surprised this wasn't supported already, but the
implementation is simple and cannot really break any existing
functionality so here it is.

src/commands/get.command.ts

@@ -82,6 +82,8 @@ export class GetCommand extends DownloadCommand {
                 return await this.getUri(id);
             case 'totp':
                 return await this.getTotp(id);
+            case 'notes':
+                return await this.getNotes(id);
             case 'exposed':
                 return await this.getExposed(id);
             case 'attachment':
@@ -239,6 +241,22 @@ export class GetCommand extends DownloadCommand {
         return Response.success(res);
     }
 
+    private async getNotes(id: string) {
+        const cipherResponse = await this.getCipher(id,
+            c => !Utils.isNullOrWhitespace(c.notes));
+        if (!cipherResponse.success) {
+            return cipherResponse;
+        }
+
+        const cipher = cipherResponse.data as CipherResponse;
+        if (Utils.isNullOrWhitespace(cipher.notes)) {
+            return Response.error('No notes available for this item.');
+        }
+
+        const res = new StringResponse(cipher.notes);
+        return Response.success(res);
+    }
+
     private async getExposed(id: string) {
         const passwordResponse = await this.getPassword(id);
         if (!passwordResponse.success) {