core: Support a per-remote "proxy" configuration option

We don't want to have to force people to set it in the environment.

https://bugzilla.gnome.org/show_bug.cgi?id=733734

doc/ostree.repo-config.xml

@@ -126,6 +126,12 @@ Boston, MA 02111-1307, USA.
         <literal>https</literal>.</para></listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><varname>proxy</varname></term>
+        <listitem><para>A string value, if given should be a URL for a
+        HTTP proxy to use for access to this repository.</para></listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><varname>gpg-verify</varname></term>
         <listitem><para>A boolean value, defaults to true.