[tumblr] replace '-' with ' ' in tag searches (fixes #611)

To search for tags with actual minus signs in them
(there shouldn't be too many,) manually replace those
with url-encoded minus characters ('-' -> '%2d')
before inputting them into gallery-dl:

https://s679874.tumblr.com/tagged/tag-with-minus
 ->
https://s679874.tumblr.com/tagged/tag%2dwith%2dminus

gallery_dl/extractor/tumblr.py

@@ -310,7 +310,7 @@ class TumblrTagExtractor(TumblrExtractor):
 
     def __init__(self, match):
         TumblrExtractor.__init__(self, match)
-        self.tag = text.unquote(match.group(3))
+        self.tag = text.unquote(match.group(3).replace("-", " "))
 
     def posts(self):
         return self.api.posts(self.blog, {"tag": self.tag})