`std.string.strip` now accepts a string of characters to be stripped

New overload functions of $(REF strip, std, string),
$(REF stripLeft, std, string) and
$(REF stripRight, std, string) accepts a string of characters to be
stripped instead of stripping only whitespace.

---
import std.string: stripLeft, stripRight, strip;

assert(stripLeft("www.dlang.org", "w.") == "dlang.org");
assert(stripRight("dlang.org/", "/") == "dlang.org");
assert(strip("www.dlang.org/", "w./") == "dlang.org");
---
