feat(template): add `lang` YAML front-matter data

BREAKING CHANGE: Now you can define `lang` tag for your post, simply add `lang` to your post front-matter data, for example:

```
lang: ar
```

then define your own styles in `custom.less`:

```css
[lang=ar] {
  direction: rtl;
}
```
Merge branch 'master' of github.com:sparanoid/sparanoid.com

* 'master' of github.com:sparanoid/sparanoid.com:
  feat(template): add `lang` YAML front-matter data