Adds the possibility to use the inverted default colors for per-sign colors

Adds code for getting inverted colors if "inverted" is given as the per-sign color
Fixes exceptions thrown when the per-sign config value is malformed
Changes the default to use inverted colors for crimson, dark_oak, spruce and warped signs
Adds methods to get main and highlighting colors from the portal sign drawer in case the given default colors are invalid and fallback colors are necessary instead

src/main/java/net/knarcraft/stargate/portal/PortalSignDrawer.java

@@ -85,6 +85,24 @@ public class PortalSignDrawer {
         PortalSignDrawer.perSignHighlightColors = signHighlightColors;
     }
 
+    /**
+     * Gets the currently used main sign color
+     *
+     * @return <p>The currently used main sign color</p>
+     */
+    public static ChatColor getMainColor() {
+        return mainColor;
+    }
+
+    /**
+     * Gets the currently used highlighting sign color
+     *
+     * @return <p>The currently used highlighting sign color</p>
+     */
+    public static ChatColor getHighlightColor() {
+        return highlightColor;
+    }
+
     /**
      * Draws the sign of the portal this sign drawer is responsible for
      */