UseUTF8 rule should be case insensitive

[pdehaan]

Related to #4, it seems that the UseUTF8 rule isn't case sensitive:

➜  idea-town git:(master) ✗ jadelint views/**/*.jade

views/includes/header.jade
  ✖  line 2  Only use UTF-8 as charset

✖  1 warning

➜  idea-town git:(master) ✗ cat views/includes/header.jade
head
  meta(charset="utf-8")
  meta(name="description" content=desc)
  meta(name="keywords" content=keys)
  meta(name="viewport" content="width=device-width initial-scale=1")
  title= title

  link(rel="stylesheet" href="https://fonts.googleapis.com/css?family=Open+Sans:400,300")
  link(rel="stylesheet" href="styles/main.min.css")
``

[rrdelaney]

Done and published