Add ability to bypass browser cache when fetching a resource

Related issue:
- https://github.com/uBlockOrigin/uBlock-issues/issues/682#issuecomment-515197130

The following advanced setting has been added:

    updateAssetBypassBrowserCache

Default to `false`. If set to `true`, uBO will ensure the
browser cache is bypassed when fetching a remote resource.

This is for the convenience of filter list maintainers who
may want to test the latest version of their lists when
fetched from their remote location.

src/js/assets.js

@@ -69,8 +69,16 @@ api.fetchText = function(url, onLoad, onError) {
     // https://github.com/gorhill/uBlock/issues/2592
     //   Force browser cache to be bypassed, but only for resources which have
     //   been fetched more than one hour ago.
+    //
+    // https://github.com/uBlockOrigin/uBlock-issues/issues/682#issuecomment-515197130
+    //   Provide filter list authors a way to completely bypass
+    //   the browser cache.
     if ( isExternal ) {
-        const queryValue = `_=${Math.floor(Date.now() / 3600000) % 12}`;
+        const cacheBypassToken =
+            µBlock.hiddenSettings.updateAssetBypassBrowserCache
+                ? Math.floor(Date.now() /    1000) % 86400
+                : Math.floor(Date.now() / 3600000) %    12;
+        const queryValue = `_=${cacheBypassToken}`;
         if ( actualUrl.indexOf('?') === -1 ) {
             actualUrl += '?';
         } else {

src/js/background.js

@@ -58,6 +58,7 @@ const µBlock = (function() { // jshint ignore:line
         selfieAfter: 3,
         strictBlockingBypassDuration: 120,
         suspendTabsUntilReady: 'unset',
+        updateAssetBypassBrowserCache: false,
         userResourcesLocation: 'unset',
     };