Add option to ignore other configuration

If `ignoreOtherConfiguration` is used JSON configurations should be
ignored.

Part of https://github.com/hypothesis/viahtml/pull/192

src/annotator/config/settings.js

 import { parseJsonConfig } from '../../boot/parse-json-config';
+import { toBoolean } from '../../shared/type-coercions';
 
 import configFuncSettingsFrom from './config-func-settings-from';
 import isBrowserExtension from './is-browser-extension';
@@ -20,9 +21,18 @@ import isBrowserExtension from './is-browser-extension';
  * @return {SettingsGetters}
  */
 export default function settingsFrom(window_) {
-  const jsonConfigs = parseJsonConfig(window_.document);
+  // Prioritize the `window.hypothesisConfig` function over the JSON format
+  // Via uses `window.hypothesisConfig` and makes it non-configurable and non-writable.
+  // In addition, Via sets the `ignoreOtherConfiguration` option to prevent configuration merging.
   const configFuncSettings = configFuncSettingsFrom(window_);
 
+  let jsonConfigs;
+  if (toBoolean(configFuncSettings.ignoreOtherConfiguration)) {
+    jsonConfigs = {};
+  } else {
+    jsonConfigs = parseJsonConfig(window_.document);
+  }
+
   /**
    * Return the href of the first annotator link in the given
    * document with this `rel` attribute.

src/annotator/config/test/settings-test.js

@@ -580,9 +580,18 @@ describe('annotator/config/settingsFrom', () => {
         specify: 'it returns setting values from window.hypothesisConfig()',
         isBrowserExtension: false,
         configFuncSettings: { foo: 'configFuncValue' },
-        jsonSettings: {},
+        jsonSettings: { foo: 'ignored' }, // hypothesisConfig() overrides js-hypothesis-config
         expected: 'configFuncValue',
       },
+      {
+        when: 'the client is embedded in a web page',
+        specify:
+          'it ignores settings from js-hypothesis-config if `ignoreOtherConfiguration` is present',
+        isBrowserExtension: false,
+        configFuncSettings: { ignoreOtherConfiguration: '1' },
+        jsonSettings: { foo: 'ignored' },
+        expected: null,
+      },
       {
         when: 'the client is embedded in a web page',
         specify: 'it returns setting values from js-hypothesis-config objects',