Don't log javascript console messages by default.

doc/help/settings.asciidoc

@@ -19,6 +19,7 @@
 |<<general-site-specific-quirks,site-specific-quirks>>|Enable workarounds for broken sites.
 |<<general-default-encoding,default-encoding>>|Default encoding to use for websites.
 |<<general-new-instance-open-target,new-instance-open-target>>|How to open links in an existing instance if a new one is launched.
+|<<general-log-javascript-console,log-javascript-console>>|Whether to log javascript console messages.
 |==============
 
 .Quick reference for section ``ui''
@@ -371,6 +372,17 @@ Valid values:
 
 Default: +pass:[window]+
 
+[[general-log-javascript-console]]
+=== log-javascript-console
+Whether to log javascript console messages.
+
+Valid values:
+
+ * +true+
+ * +false+
+
+Default: +pass:[false]+
+
 == ui
 General options related to the user interface.
 

qutebrowser/browser/webpage.py

@@ -310,7 +310,8 @@ class BrowserPage(QWebPage):
 
     def javaScriptConsoleMessage(self, msg, line, source):
         """Override javaScriptConsoleMessage to use debug log."""
-        log.js.debug("[{}:{}] {}".format(source, line, msg))
+        if config.get('general', 'log-javascript-console'):
+            log.js.debug("[{}:{}] {}".format(source, line, msg))
 
     def chooseFile(self, _frame, suggested_file):
         """Override QWebPage's chooseFile to be able to chose a file to upload.

qutebrowser/config/configdata.py

@@ -189,6 +189,10 @@ DATA = collections.OrderedDict([
          SettingValue(typ.NewInstanceOpenTarget(), 'window'),
          "How to open links in an existing instance if a new one is "
          "launched."),
+
+        ('log-javascript-console',
+         SettingValue(typ.Bool(), 'false'),
+         "Whether to log javascript console messages."),
     )),
 
     ('ui', sect.KeyValue(