Added RTL mixin

This mixin allows you to put your RTL code within the main stylesheet, rather than in a separate one, by taking advantage of the 'dir' attribute added by Wordpress via the 'language_attributes()' function.

I previously used the 'rtl' class that Wordpress adds to the body, but this makes it easier to target all elements on the page including the body tag.

As an example

```sass
body {
  @include rtl {
    direction: rtl;
    unicode-bidi: embed;
  }
}
```

compiles as

```css
html[dir="rtl"] body {
    direction: rtl;
    unicode-bidi: embed;
}
```

But you can use it for other things, like floats or alignments like

```sass
.comment {
  .avatar {
    float: left;
    @include rtl {
      float: right;
    }
  }
}
```

compiles as

```css
.comment .avatar {
  float: left;
}

html[dir="rtl"] .comment .avatar {
  float: right;
}
```