The field has three option :
- lesser or equal than. If null, this option is ignored ;
- greather or equal than. If null, this options is ignored ;
- precision : the number of decimal after the number ;
- text after the field : a text to show after the field.
The field is rendered as an HTML integer input if precision = 0, or a
symfony number field if precision > 0.
ref chill-project/Chill-CustomFields#11
