std.getopt.config: change type to disallow or support bitwise operands

[dlangBugzillaToGithub]

dhasenan (@dhasenan) reported this on 2018-11-03T18:37:33Z

Transfered from https://issues.dlang.org/show_bug.cgi?id=19362

Description

Test case:
---
unittest
{
  import std.getopt;
  auto args = ["exe", "-h"];
  int intFlag = 5;
  auto result = getopt(args, config.caseSensitive | config.bundling, "H", &intFlag);
  assert(intFlag == 5);
  assert(result.helpWanted);
}
---

It's normal with logically composable enums to be able to or them together. This doesn't work for std.getopt. If it compiles, it should do the right thing.