Add standalone implementation

Add standalone implementation, giving a new choice to avoid using deprecated libgnome-keyring

README.md

@@ -30,16 +30,16 @@ Secondly, choose an implementation: `standalone` impl only allows to unlock defa
 
 <details>
   <summary>Standalone Implementation</summary>
-```
-cd gnome-keyring-yubikey-unlock/src && make KEYRING_IMPL=standalone && cd ..
-```
+  ```
+  cd gnome-keyring-yubikey-unlock/src && make KEYRING_IMPL=standalone && cd ..
+  ```
 </details>
 
 <details>
   <summary>Lib Implementation</summary>
-```
-cd gnome-keyring-yubikey-unlock/src && make KEYRING_IMPL=lib && cd ..
-```
+  ```
+  cd gnome-keyring-yubikey-unlock/src && make KEYRING_IMPL=lib && cd ..
+  ```
 
 ### Extra Dependency for "lib" implementation