programs.khal: ability to set RGB color (#5192)

The current module constrains to values in enum but khal supports RGB
colors as well
khal.readthedocs.io/en/latest/configure.html#the-calendars-section !
(be careful when setting an RGB value, it has to be quoted else it is ignored, got bitten by it with a manual config )

NB: It's also not possible to set addresses khal.readthedocs.io/en/latest/configure.html#the-calendars-section

modules/programs/khal-accounts.nix

@@ -15,27 +15,11 @@ with lib;
     };
 
     color = mkOption {
-      type = types.nullOr (types.enum [
-        "black"
-        "white"
-        "brown"
-        "yellow"
-        "dark gray"
-        "dark green"
-        "dark blue"
-        "light gray"
-        "light green"
-        "light blue"
-        "dark magenta"
-        "dark cyan"
-        "dark red"
-        "light magenta"
-        "light cyan"
-        "light red"
-      ]);
+      type = types.nullOr types.str;
       default = null;
       description = ''
         Color in which events in this calendar are displayed.
+        For instance 'light green' or an RGB color '#ff0000'
       '';
       example = "light green";
     };

modules/programs/khal.nix

@@ -45,10 +45,11 @@ let
         + (optionalString
           (value.khal.type == "birthdays" && value.khal ? thisCollection)
           value.khal.thisCollection)
-      }"
-    ] ++ optional (value.khal.readOnly) "readonly = True" ++ [
-      (toKeyValueIfDefined (getAttrs [ "type" "color" "priority" ] value.khal))
-    ] ++ [ "\n" ]);
+      }\n        "
+    ] ++ optional (value.khal.readOnly) "readonly = True"
+      ++ optional (value.khal.color != null) "color = '${value.khal.color}'"
+      ++ [ (toKeyValueIfDefined (getAttrs [ "type" "priority" ] value.khal)) ]
+      ++ [ "\n" ]);
 
   localeFormatOptions = let
     T = lib.types;