Add attribute for setting scroller interpolator

Created by: kiyeonk

This is for solving #466 (closed)

• res/anim/overshoot.xml

<?xml version="1.0" encoding="utf-8"?>
<overshootInterpolator xmlns:android="http://schemas.android.com/apk/res/android"
    android:tension="1.1" />

• res/layout/activity_demo.xml

<com.sothree.slidinguppanel.SlidingUpPanelLayout
    ...
    sothree:umanoScrollInterpolator="@anim/overshoot">
...