Allow adjusting the final position of .place() method via callback

[niklasramo]

• Allow using a function for maximum extendibility.
• Add new collision action: "flip".
• Add new collision action: "elemX elemY targetX targetY". This would be a fallback position.

[niklasramo]

Actually thinking about this more it might be better to allow user to provide a function for the contain.onCollision option. I tried to replace popo.js in a project and it turned out to be a lot of work in the scenarios where popo's collision callback was used.

To provide more insight, this is the scenario I had:

popo($target[0], 'get', {
  base: $item[0],
  position: 'right top right bottom',
  container: window,
  collision: function (pos, overlap) {
    if(overlap.bottom < 0) {
      $target.addClass('reverse');
    }
  }
});

Replicating this with current version of Mezr requires a lot more work.

So, let's add the callback to Mezr also and make it behave similarly to that of popo's: https://github.com/niklasramo/popo.js#example-3.

The other collision actions are just a nice bonus after the callback function is implemented since you can pretty easily build any kind of collision handling behavior within the callback.

[niklasramo]

So after thinking about this more I think it's best to leave onCollision as it it and provide a new optional callback as an option, which is can be used to modify the return value of the method and which also receives all the available positioning data. This way user's can use the available collision methods and additionally modify the final result after collision correction.