Title in front matter overrides theme config

@ulivz @yyx990803 does this cause a double up in the title meta tag. 

If I have...

```
module.exports = {
  title: 'Theme Title',
}
```

and

```
---
layout: about
title: About Us
---
```

I get `<title>About Us | About Us</title>`

Whereas, if I have:

```
---
layout: about
---

About Us
```

I get `<title>About Us | Theme Title</title>`