common: add iterator-based string splitter

common: add iterator-based string splitter

ranged-for loop example:

  for (std::string_view s : split(input)) {
    ...

container initialization example:

  auto parts = split(input);

  std::vector<std::string> strings;
  strings.assign(parts.begin(), parts.end());

Signed-off-by: Casey Bodley <cbodley@redhat.com>