Allow theme config to be overridden by site config

If there is a `theme_config` object in the main site config, it is merged into
the theme's config.

Ex:
```yml
\# _config.yml
theme_config:
  bio: "My awesome bio"
```

```yml
\# themes/my-theme/_config.yml
bio: "Some generic bio"
logo: "a-cool-image.png"
```

results in the following theme config
```js
{
  bio: "My awesome bio",
  logo: "a-cool-image.png"
}
```