Support custom initial drawer scale

Allows customizing how much the drawer segment scales.
More importantly, enables disabling this scale effect completely if desired
to have more simple push drawer.

Relates to alex-melnyk#52

lib/src/widget.dart

@@ -11,6 +11,7 @@ class AdvancedDrawer extends StatefulWidget {
     this.backdrop,
     this.openRatio = 0.75,
     this.openScale = 0.85,
+    this.initialDrawerScale = 0.75,
     this.animationDuration = const Duration(milliseconds: 250),
     this.animationCurve,
     this.childDecoration,
@@ -41,6 +42,10 @@ class AdvancedDrawer extends StatefulWidget {
   /// Opening ratio.
   final double openScale;
 
+  /// How large the drawer segment should scale from.
+  /// Set to 1 to disable drawer segment scale effect.
+  final double initialDrawerScale;
+
   /// Animation duration.
   final Duration animationDuration;
 
@@ -228,7 +233,7 @@ class _AdvancedDrawerState extends State<AdvancedDrawer>
           );
 
     _drawerScaleAnimation = Tween<double>(
-      begin: 0.75,
+      begin: widget.initialDrawerScale,
       end: 1.0,
     ).animate(parentAnimation);