Added configtype to wrap valid prefixes.

qutebrowser/config/configtypes.py

@@ -142,6 +142,27 @@ class ValidValues:
                 self.descriptions == other.descriptions)
 
 
+class ValidPrefixes(ValidValues):
+
+    def __init__(self, *values, separator: str = ':', **kwargs):
+        super().__init__(*values, **kwargs)
+        self.separator = separator
+
+    def __contains__(self, item) -> bool:
+        return any(map(lambda x: item.startswith(x + self.separator), self.values))
+
+    def __iter__(self) -> Iterator[str]:
+        return map(lambda x: x + self.separator, super().__iter__())
+
+    def __repr__(self) -> str:
+        return utils.get_repr(self, values=self.values, separator=self.separator,
+                              descriptions=self.descriptions)
+
+    def __eq__(self, other: object) -> bool:
+        assert (isinstance(other, ValidPrefixes))
+        return super().__eq__(other) and self.separator == other.separator
+
+
 class BaseType:
 
     """A type used for a setting value.