polybar: single-quote instead of double

Polybar expects quoted values only when whitespace is important to the value

Fixes #356

modules/services/polybar.nix

@@ -10,10 +10,15 @@ let
 
   toPolybarIni = generators.toINI {
     mkKeyValue = key: value:
-    let
+   let
+      quoted = v:
+        if hasPrefix " " v || hasSuffix " " v
+        then ''"${v}"''
+        else v;
+
       value' =
         if isBool value then (if value then "true" else "false")
-        else if (isString value && key != "include-file") then ''${value}''
+        else if (isString value && key != "include-file") then quoted value
         else toString value;
     in
       "${key}=${value'}";