Add support for -stdin-filename for the Crystal linter

The Crystal linter [now supports](https://github.com/aurimasniekis/crystal/commit/59039470eab77e604256b5228ca95100ee7e3403) an `-stdin-filename` flag which makes it possible to lint buffers while you type. An option should be added for enabling this behaviour, with documentation which states that enabling the option will require restarting Vim, and support could be automatic after enough months have passed.