refactor(segment): cleaned up both iOS and md mode styling

Converted values to some sass variables, moved transition to after
button tap, added border bottom for md mode, reorganized the sass
variables by the order they’re used, removed the use of !important,
added iOS hover background-color. Closes #283

ionic/components/segment/test/basic/index.ts

@@ -13,7 +13,8 @@ class MyApp {
     this.myForm = fb.group({
       mapStyle: ['hybrid', Validators.required]
     });
-    console.log(this.myForm);
+
+    this.modelStyle = 'B';
   }
 
   doSubmit(event) {