Allow different behavior for directions, and hold after completion

[gtg922r]

The best way to demonstrate the point of these changes is to illustrate the final behavior:

Allow different behavior for left or right swipe

• Added thirdTrigger and fourthTrigger to allow different trigger point for right to left swiping (6a08db2). If unset, will just use firstTrigger and secondTrigger, respectively (cae4f3e)
• Added rightDefaultColor to set a different defaultColor for the start of a right to left swipe. By default it will just use defaultColor (e35fc13, fee9a61)

Allow swipe cell to completion, but then continue to allow interaction

• I wanted to be able to use MCSwipeTableViewCell to add a layer of interaction "below" the cell (either for editing functions, or additional detail).
• MCSwipeTableViewCellModeExit no longer allow interaction with the cell or to swipe it back to the origin
• MCSwipeTableViewCellModeSwitch obviously won't swipe to completion
• Added a new mode MCSwipeTableViewCellModeHold to enable this behavior (see the gif above)

Add additional delegate callback that includes the state at the time of completion

• Seems like a useful argument to add to the callback method. Don't want to break existing code, so added it as an extra method

Note about animation

• The gif above also includes changes to the animations. Those aren't generic and are not included in a pull request. If you use only an icon, everything would work fine. If you use a full width uiview, then the animation is wrong.
• If you want to accept this pull request, and I figure out a way to make the changes generic I'll follow up with another pull request