This commit introduces a new feature called `"regex"`. It adds a new
function `validator_regex` to `Arg` and was inspired by the discussion
in #1968. The name `validator_regex` was chosen instead of
`regex_validator` to make sure that the developer keeps in mind that
there may only be a single `Validator` on an `Arg`.
The feature can be used with YAML files, however there is no proper
pattern in `clap_app!` (yet).
