Add hook to selection

[mankowitz]

I added an overwritable function called processItem that allows the user to process the mention before inserting it in the element

[fritx]

@mankowitz good idea.

I guess a corresponding deleteMatch should be defined to use together with processItem??

  deleteMatch: {
    type: Function,
    default: (name, chunk, suffix) => {
      return chunk === name + suffix
    }
  },

[mankowitz]

@fritx - Can you merge the processItem into the next build?

[fritx]

@mankowitz sorry.. but have a look at my previous comments :D

The changes seem to be more than just one feature, I want things to be nailed in certain commits.

And also, I would switch to slot-scope in a separated commit and bump since it would no longer work with Vue 2.4.x

[fritx]

@mankowitz i'm sorry you have deleted the source of this PR and also there are some conflicts in it that I can not merge it right now.