[enh] engine blacklist support

searx/engines/__init__.py

 from itertools import izip_longest, chain
 from operator import itemgetter
 from urlparse import urlparse
+from searx import settings
 
 engine_dir = dirname(realpath(__file__))
 
 categories = {'general': []}
 
 for filename in listdir(engine_dir):
-    modname = splitext(filename)[0]
     if filename.startswith('_') or not filename.endswith('.py'):
         continue
+    modname = splitext(filename)[0]
+    if modname in settings.blacklist:
+        continue
     filepath = join(engine_dir, filename)
     engine = load_source(modname, filepath)
     engine.name = modname