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).
The YAML null value is called `null` in YAML, not `Null`. yaml-rust
handles those values according to spec, so we should use the correct
capitalization.
See https://yaml.org/spec/1.2/spec.html#id2803362 for more
information.
Adds new method/attribute `Arg::value_hint`, taking a `ValueHint` enum
as argument. The hint can denote accepted values, for example: paths,
usernames, hostnames, commands, etc.
This initial implementation supports hints for the zsh and fish
completion generators, support for other shells can be added later.
this implements Arg::short_alias, Arg::short_aliases,
Arg::short_visible_alias, and Arg::short_visible_aliases in addition to
adding their associated tests
