support arbitrary source URLs

This allows the user to specify a different repository, possibly
outside of GitHub to link the "show source" to.

Closes: #87

Author: anarcat

alabaster/layout.html

@@ -51,9 +51,14 @@
       {% endif %}
       {%- if show_source and has_source and sourcename %}
       {% if show_copyright or theme_show_powered_by %}|{% endif %}
+      {%- if source_url_prefix %}
+      <a href="{{ source_url_prefix }}{{ pagename }}{{ source_suffix }}"
+          rel="nofollow">{{ _('Page source') }}</a>
+      {%- else %}
       <a href="{{ pathto('_sources/' + sourcename, true)|e }}"
           rel="nofollow">{{ _('Page source') }}</a>
       {%- endif %}
+      {%- endif %}
     </div>
 
     {% if theme_github_banner|lower != 'false' %}

alabaster/theme.conf

@@ -28,6 +28,7 @@ sidebar_includehidden = true
 sidebar_collapse = true
 show_powered_by = true
 show_related = false
+source_url_prefix =
 
 gray_1 = #444
 gray_2 = #EEE

docs/changelog.rst

@@ -2,6 +2,8 @@
 Changelog
 =========
 
+* :feature:`87` Add support for arbitrary "edit source" link
+  targets. Credit: ``@anarcat``.
 * :support:`- backported` Miscellaneous project maintenance updates such as
   adding to Travis CI and enforcing the use of ``flake8``.
 * :feature:`110 backported` Add ``badge_branch`` option allowing