Multiple additional items on each side.

Ability to specify how many items should be visible on each side of currently selected item.

Value should be assignable through xml
```xml
app:numSideItems="1"
```

or from java,
```java
horizontalPicker.setNumSizeItems(1);
```

Or something like that. By default it should be set to 1 so it won't break apps when upgrading.