Add extra fields to Kirby users

user.yml

fields: 
  twitter: 
    label: Twitter
    type: text
    placeholder: @username
    icon: twitter
    width: 1/2
  instagram: 
    label: Instagram
    type: text
    placeholder: Instagram username
    icon: instagram
    width: 1/2  
  linkedin: 
    label: Linkedin
    type: text
    placeholder: Linkedin username
    icon: linkedin
    width: 1/2  
  tumblr: 
    label: Tumblr
    type: text
    placeholder: Tumblr username
    icon: tumblr
    width: 1/2  
  github: 
    label: Github
    type: text
    placeholder: Github username
    icon: github
    width: 1/2