[less mode] improve highlighting, indentation

Better handling of attributes, .2px, tags.
````
.page {
  .content {
    width: 870px;
    padding: 45px;
  }
  margin: 0 auto;
  font-family: 'Georgia', serif;
  font-size: 18px;
  line-height: 26px;
  padding: 0 60px;
  code {
    font-size: 16px;
  }
  pre {
    border-width: 1px;
    border-style: dashed;
    padding: 15px;
    margin: 15px 0;
  }
  h1 {
    text-align: left;
    font-size: 40px;
    margin-top: 15px;
    margin-bottom: 35px;
  }
  p + h1 { margin-top: 60px }
  h2, h3 {
    margin: 30px 0 15px 0;
  }
  p + h2, pre + h2, code + h2 {
    border-top: 6px solid rgba(255, 255, 255, 0.1);
    padding-top: 30px;
  }
  h3 {
    margin: 15px 0;
  }
}
````