Check for site github_brance (#43)

Otherwise default to master.

_includes/editable.html

@@ -1,4 +1,4 @@
-<a href="{{ site.repo }}/edit/{{ site.github_branch }}/{{ page.path }}" target="_blank"><i class="fa fa-edit fa-fw"></i> Edit this page</a>
+<a href="{{ site.repo }}/edit/{% if site.github_branch %}{{ site.github_branch }}{% else %}master{% endif %}/{{ page.path }}" target="_blank"><i class="fa fa-edit fa-fw"></i> Edit this page</a>
 <a href="{{ site.repo }}/issues/new?labels={% if page.editable %}{{ page.editable }}{% else %}question{% endif %}&title=Question:&body=Question on: {{ site.repo }}/tree/master/{{ page.path }}" target="_blank"><i class="fab fa-github fa-fw"></i> Create documentation issue</a>
 <a href="{{ site.repo }}/issues/new" target="_blank"><i class="fas fa-tasks fa-fw"></i> Create project issue</a>
 <!-- this will parse through the header fields and add a button to open