Make preventing the default touch event optional.

[sennett]

Adds a preventDefaultTouchMoveEvent prop to Swipeable that looks like this:

<Swipeable
    onSwiping={this.swiping}
    ...
    preventDefaultTouchMoveEvent={false}>
    <div>
        This element can be swiped
    </div>
</Swipeable>

The default is true to preserve default behaviour.

This PR supersedes PR #1 and resolves #28 and #21 (as long as the option is passed).

• readme updated
• examples updated.

Thanks.

[hartzis]

Thank you for this. I think we should be able to get this in.

I think I'm also going to get a PR in to update the cancelPageSwipe inside the touchMove to also add checks for onSwiped callbacks to solve for #21, specifically fruitkiwi's comment.