Select days by clicking day-labels

[bjarnef]

Make it possible to select days by clicking labels either by use forattribute on label + id on input.

<label for="some-unque-id">Monday</label>
<input type="checkbox" value="1" id="some-unque-id" />

or wrap input inside label:

<label>Monday
    <input type="checkbox" value="1"  />
</label>

For this PR for extending datepicker options in Umbraco, I added a prevalue editor for weekdays: umbraco/Umbraco-CMS#984

https://github.com/bjarnef/Umbraco-CMS/blob/dev-v7-U4-7301/src/Umbraco.Web.UI.Client/src/views/propertyeditors/datepicker/weekdays.prevalues.html

https://github.com/bjarnef/Umbraco-CMS/blob/dev-v7-U4-7301/src/Umbraco.Web.UI.Client/src/views/propertyeditors/datepicker/weekdays.controller.js

Umbraco already use checklist-model, so that is pretty easy :) http://vitalets.github.io/checklist-model/