Fix _config.yml for customizable title, description

[ryanking13]

On #17, @frigaut asked about how to change the site's title, since it always shows a same description "Simplistic, responsive jekyll based open source theme", and you said it's not yet customizable.

I fixed _config.yml so that jekyll-seo-tag can automatically generates site's custom title and description properly.

---

In _includes/head.html, actually there is a code that generates site's custom title and description from _config.yml's theme_settings:title, theme_settings:title but it's manually disabled by commenting out 4 months ago.

    <!-- Manual seo tags -->
    <!--
    <title>{% if page.title %}{{ page.title }} |{% endif %} {{ site.theme_settings.title }}</title>
    <meta name="description" content="{% if page.excerpt %}{{ page.excerpt | strip_html | strip_newlines | truncate: 160 }}{% else %}{{ site.theme_settings.description }}{% endif %}">
    -->

I'm not sure you intended that for later implementation, if you did, please ignore this PR.

[sylhare]

Indeed, thanks ;)