add '--items' option

this allows to specify which manga-chapters/comic-issues to download
when using gallery-dl on a manga/comic URL

gallery_dl/util.py

@@ -0,0 +1,47 @@
+# -*- coding: utf-8 -*-
+
+# Copyright 2017 Mike Fährmann
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 2 as
+# published by the Free Software Foundation.
+
+"""Utility functions"""
+
+
+def apply_range(iterable, rangespec):
+    """Return a new iterable containing only the items specified in the given
+    integer range
+    """
+    try:
+        maxval = len(iterable)
+    except TypeError:
+        maxval = 0
+    rset = parse_range(rangespec, maxval)
+    return (
+        item
+        for index, item in enumerate(iterable, 1)
+        if index in rset
+    )
+
+
+def parse_range(rangespec, maxval=0):
+    """Parse an integer range and return the resulting set
+
+    Examples
+        parse_range("-2,4,6-8,10-", 12)  -> set(1, 2, 4, 6, 7, 8, 10, 11, 12)
+        parse_range(" - 3 , 4-  4, 6-2") -> set(1, 2, 3, 4)
+    """
+    result = set()
+    for group in rangespec.split(","):
+        parts = group.split("-", maxsplit=1)
+        try:
+            if len(parts) == 1:
+                result.add(int(parts[0]))
+            else:
+                beg = int(parts[0]) if parts[0].strip() else 1
+                end = int(parts[1]) if parts[1].strip() else maxval
+                result.update(range(beg, end+1))
+        except ValueError:
+            pass
+    return result