* feat(arg_value): ArgValue can be used for possible_values
Through the ArgValue it is possible:
* `hide` possible_values from showing in completion, help and validation
* add `about` to possible_values in completion
* Resolved a few change-requests by epage
* make clippy happy
* add ArgValue::get_visible_value
* remove verbose destructering
* rename ArgValue::get_hidden to ArgValue::is_hidden
* add test for help output of hidden ArgValues
* Documentation for ArgValue
There is an issue that required to implement From<&ArgValue> for
ArgValue. We should probably find a solution without that.
* fix requested changes by epage
* fix formatting
* add deref in possible_values call to remove From<&&str>
* make clippy happy
* use copied() instad of map(|v|*v)
* Finishing up for merge, hopefully
* changes requested by pksunkara
