add '--filter' command-line option

This allows for image filtering via Python expressions by the same
metadata that is also used to build filenames (--list-keywords).

The usually shunned eval() function is used to evaluate
filter-expressions, but it seemed quite appropriate in this case and
shouldn't introduce any new security issues, as any attacker that could do
> gallery-dl --filter "delete-everything()" ...
could as well do
> python -c "delete-everything()"

gallery_dl/util.py

@@ -150,8 +150,8 @@ def build_predicate(predicates):
 
 class RangePredicate():
     """Predicate; True if the current index is in the given range"""
-    def __init__(self, rangespec):
-        self.ranges = optimize_range(parse_range(rangespec))
+    def __init__(self, ranges):
+        self.ranges = ranges
         self.index = 0
         if self.ranges:
             self.lower, self.upper = self.ranges[0][0], self.ranges[-1][1]
@@ -182,6 +182,20 @@ class UniquePredicate():
         return False
 
 
+class FilterPredicate():
+    """Predicate; True if evaluating the given expression returns True"""
+    globalsdict = {"__builtins__": {}}
+
+    def __init__(self, codeobj):
+        self.codeobj = codeobj
+
+    def __call__(self, url, kwds):
+        try:
+            return eval(self.codeobj, self.globalsdict, kwds)
+        except Exception as exc:
+            raise exception.FilterError(exc)
+
+
 class ChainPredicate():
     """Predicate; True if all of its predicates return True"""
     def __init__(self, predicates):