Edit fields functionality

[rGiladi]

This functionality should help solve a scenario where you want to use one of the default fields but to change one of its properties.

I was thinking on a direction of providing the ability to edit the field's array directly. Another possibility is to take advantage of deepmerge's arrayMerge option and provide a simple algorithm that will merge arrays in a way the we can do something like

AccountsReact.editFields('signUp', [
  {
     _id: 'password',
     maxLength: 72
  }
])