includes: Enhance Twitter share link (#538)

* Include the page title before the URL
* If twitter.username is set, pass the `via` paramter for attribution

Author: kylemanna

_includes/social-share.html

@@ -5,11 +5,11 @@
     <h4 class="page__share-title">{{ site.data.ui-text[site.locale].share_on_label | default: "Share on" }}</h4>
   {% endif %}
 
-  <a href="https://twitter.com/intent/tweet?text={{ base_path }}{{ page.url }}" class="btn btn--twitter" title="{{ site.data.ui-text[site.locale].share_on_label | default: 'Share on' }} Twitter"><i class="fa fa-fw fa-twitter" aria-hidden="true"></i><span> Twitter</span></a>
+  <a href="https://twitter.com/intent/tweet?{% if site.twitter.username %}via={{ site.twitter.username }}&{% endif %}text={{ page.title }} {{ base_path }}{{ page.url }}" class="btn btn--twitter" title="{{ site.data.ui-text[site.locale].share_on_label | default: 'Share on' }} Twitter"><i class="fa fa-fw fa-twitter" aria-hidden="true"></i><span> Twitter</span></a>
 
   <a href="https://www.facebook.com/sharer/sharer.php?u={{ base_path }}{{ page.url }}" class="btn btn--facebook" title="{{ site.data.ui-text[site.locale].share_on_label | default: 'Share on' }} Facebook"><i class="fa fa-fw fa-facebook" aria-hidden="true"></i><span> Facebook</span></a>
 
   <a href="https://plus.google.com/share?url={{ base_path }}{{ page.url }}" class="btn btn--google-plus" title="{{ site.data.ui-text[site.locale].share_on_label | default: 'Share on' }} Google Plus"><i class="fa fa-fw fa-google-plus" aria-hidden="true"></i><span> Google+</span></a>
 
   <a href="https://www.linkedin.com/shareArticle?mini=true&url={{ base_path }}{{ page.url }}" class="btn btn--linkedin" title="{{ site.data.ui-text[site.locale].share_on_label | default: 'Share on' }} LinkedIn"><i class="fa fa-fw fa-linkedin" aria-hidden="true"></i><span> LinkedIn</span></a>
-</section>
+</section>