Whitelist and document the 'services' config param

Document the 'services' config param which the host page may provide to
the client.

docs/config.md

@@ -34,3 +34,10 @@ _Boolean_. Controls whether the sidebar opens automatically on startup.
 
 _Boolean_. Controls whether the in-document highlights are shown by default.
 (Default: _true_.)
+
+### `services`
+
+_Array_. A list of annotation services which the client should retrieve
+annotations from, optionally including information about the identity of the
+user on that service. When omitted, the client will default to connecting to
+the public [Hypothesis](https://hypothes.is/) service.

src/sidebar/host-config.js

@@ -26,6 +26,7 @@ function hostPageConfig(window) {
     'openLoginForm',
     'openSidebar',
     'showHighlights',
+    'services',
   ];
 
   return Object.keys(config).reduce(function (result, key) {

src/sidebar/test/host-config-test.js

@@ -18,6 +18,9 @@ describe('hostPageConfig', function () {
       openSidebar: true,
       openLoginForm: true,
       showHighlights: true,
+      services: [{
+        authority: 'hypothes.is',
+      }],
     });
 
     assert.deepEqual(hostPageConfig(window_), {
@@ -26,6 +29,9 @@ describe('hostPageConfig', function () {
       openSidebar: true,
       openLoginForm: true,
       showHighlights: true,
+      services: [{
+        authority: 'hypothes.is',
+      }],
     });
   });