Add donate_url and opencollective options

When either are set, adds a "Donate" section to the sidebar.
If `donate_url` is set, a "donate" shield button is used.
If `opencollective` is set, an OpenCollective button is used.

alabaster/donate.html

@@ -7,3 +7,25 @@ <h3>Donate</h3>
         src="//gttp.co/v1.js"></script>
 </p>
 {% endif %}
+
+{% if theme_donate_url or theme_opencollective %}
+<h3>Donate</h3>
+
+<p>If you find {{ project }} useful, please consider supporting the team with a donation:</p>
+
+<p>
+
+    {% if theme_donate_url %}
+    <a class="badge" href="{{ theme_donate_url }}">
+    <img src="https://img.shields.io/badge/donate-%E2%9D%A4%C2%A0-ff69b4.svg?style=flat" alt="Donate">
+    </a>
+    {% endif %}
+    {% if theme_opencollective %}
+    <a class="badge" href="https://opencollective.com/{{ theme_opencollective }}/donate" target="_blank">
+      <img src="https://opencollective.com/{{ theme_opencollective }}/donate/button.png?color={{ theme_opencollective_button_color }}" width=300 />
+    </a>
+    {% endif %}
+</p>
+
+<p>Your donation keeps {{ project }} healthy and maintained.</p>
+{% endif %}

alabaster/theme.conf

@@ -18,6 +18,9 @@ github_count = true
 badge_branch = master
 travis_button = false
 codecov_button = false
+donate_url =
+opencollective =
+opencollective_button_color = white
 gratipay_user =
 gittip_user =
 analytics_id =

docs/customization.rst

@@ -111,6 +111,13 @@ Variables and feature toggles
   string - used to display a `Codecov <https://codecov.io>`_ build status
   button in the sidebar. If ``true``, uses your ``github_(user|repo)``
   settings.
+* ``donate_url``: Set to the URL where you receive donations if you want
+  a 'Donate' section in your sidebar.
+* ``opencollective``: Set to you `OpenCollective <https://opencollective.com/>`_
+  collective name if you want an OpenCollective 'Donate' section in your
+  sidebar.
+* ``opencollective_button_color``: Set to the color for your "Donate To
+  Our Collective Button". May be ``"white"`` or ``"blue"``.
 * ``gratipay_user``: Set to your `Gratipay <https://gratipay.com>`_ username
   if you want a Gratipay 'Donate' section in your sidebar.