Specify valid loglevels for --loglevel/-l

doc/qutebrowser.1.asciidoc

@@ -66,7 +66,7 @@ show it.
     How URLs should be opened if there is already a qutebrowser instance running.
 
 === debug arguments
-*-l* 'LOGLEVEL', *--loglevel* 'LOGLEVEL'::
+*-l* '{critical,error,warning,info,debug,vdebug}', *--loglevel* '{critical,error,warning,info,debug,vdebug}'::
     Set loglevel
 
 *--logfilter* 'LOGFILTER'::

qutebrowser/qutebrowser.py

@@ -74,7 +74,9 @@ def get_argparser():
 
     debug = parser.add_argument_group('debug arguments')
     debug.add_argument('-l', '--loglevel', dest='loglevel',
-                       help="Set loglevel", default='info')
+                       help="Set loglevel", default='info',
+                       choices=['critical', 'error', 'warning', 'info',
+                                'debug', 'vdebug'])
     debug.add_argument('--logfilter',
                        help="Comma-separated list of things to be logged "
                        "to the debug log on stdout.")