core: config: add config option to enable the wayland input method

Having multiple input methods at once causes behaviour defined by the
Wayland compositor. In a recent update by Phoc, the most recently
started input method would take priority to demonstrate the instability.
Add a configuration option so the Wayland input method can be disabled
to accommodate a second input method.

Also update the README with the new option.

README.md

@@ -110,3 +110,4 @@ Configuration example:
     
     wayland:
       height: 185
+      input-method: true