Allow negating categories in --logfilter

Sometimes I want to see all the logs _except_ for the sql stuff and
"marked cookies as dirty". with this you should be able to pass
`--logfilter \!sql,save`.

doc/contributing.asciidoc

@@ -188,8 +188,8 @@ There are some useful functions for debugging in the `qutebrowser.utils.debug`
 module.
 
 When starting qutebrowser with the `--debug` flag, you also get useful debug
-logs. You can add +--logfilter _category[,category,...]_+ to restrict logging
-to the given categories.
+logs. You can add +--logfilter _[!]category[,category,...]_+ to restrict
+logging to the given categories.
 
 With `--debug` there are also some additional +debug-_*_+ commands available,
 for example `:debug-all-objects` and `:debug-all-widgets` which print a list of

qutebrowser/qutebrowser.py

@@ -146,7 +146,7 @@ def logfilter_error(logfilter):
     Args:
         logfilter: A comma separated list of logger names.
     """
-    if set(logfilter.split(',')).issubset(log.LOGGER_NAMES):
+    if set(logfilter.lstrip('!').split(',')).issubset(log.LOGGER_NAMES):
         return logfilter
     else:
         raise argparse.ArgumentTypeError(

qutebrowser/utils/log.py

@@ -182,9 +182,16 @@ def init_log(args):
     root = logging.getLogger()
     global console_filter
     if console is not None:
-        console_filter = LogFilter(None)
-        if args.logfilter is not None:
-            console_filter.names = args.logfilter.split(',')
+        if not args.logfilter:
+            negate = False
+            names = None
+        elif args.logfilter.startswith('!'):
+            negate = True
+            names = args.logfilter[1:].split(',')
+        else:
+            negate = False
+            names = args.logfilter.split(',')
+        console_filter = LogFilter(names, negate)
         console.addFilter(console_filter)
         root.addHandler(console)
     if ram is not None:
@@ -498,12 +505,14 @@ class LogFilter(logging.Filter):
     comma-separated list instead.
 
     Attributes:
-        _names: A list of names that should be logged.
+        names: A list of record names to filter.
+        negated: Wether names is a list of records to log or to suppress.
     """
 
-    def __init__(self, names):
+    def __init__(self, names, negate=False):
         super().__init__()
         self.names = names
+        self.negated = negate
 
     def filter(self, record):
         """Determine if the specified record is to be logged."""
@@ -514,12 +523,12 @@ class LogFilter(logging.Filter):
             return True
         for name in self.names:
             if record.name == name:
-                return True
+                return not self.negated
             elif not record.name.startswith(name):
                 continue
             elif record.name[len(name)] == '.':
-                return True
-        return False
+                return not self.negated
+        return self.negated
 
 
 class RAMHandler(logging.Handler):