Whitelist and document 'appType' parameter

src/sidebar/host-config.js

@@ -17,6 +17,10 @@ function hostPageConfig(window) {
     // Direct-linked annotation ID
     'annotations',
 
+    // Config param added by the extension, Via etc.  indicating how Hypothesis
+    // was added to the page.
+    'appType',
+
     // Config params documented at
     // https://github.com/hypothesis/client/blob/master/docs/config.md
     'openLoginForm',

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

@@ -14,6 +14,7 @@ describe('hostPageConfig', function () {
   it('parses config from location string and returns whitelisted params', function () {
     var window_ = fakeWindow({
       annotations: '1234',
+      appType: 'bookmarklet',
       openSidebar: true,
       openLoginForm: true,
       showHighlights: true,
@@ -21,6 +22,7 @@ describe('hostPageConfig', function () {
 
     assert.deepEqual(hostPageConfig(window_), {
       annotations: '1234',
+      appType: 'bookmarklet',
       openSidebar: true,
       openLoginForm: true,
       showHighlights: true,