Add value-keyword-case rule

[rafaelcruzazevedo]

This PR adds value-keyword-case rule.

The following patterns are considered violations:

a {
  display: Block;
}

a {
  display: bLoCk;
}

a {
  display: BLOCK;
}

a {
  transition: -WEBKIT-TRANSFORM 2s;
}

The following patterns are not considered violations:

a {
  display: block;
}

a {
  transition: -webkit-transform 2s;
}