new feature: separateValueLabel

Added the feature to enable having a separate value sent in form than from tags (useful for using this plugin to filter relational tables with primary key)

Values goes to hidden fields, and Labels goes to the tag

Autocomplete JSON should contain value, and label fields.


````
$('#tagit').tagit({
  separateValueLabel: true,
  autocomplete: {
    source: 'jsource.json'
  }
})