Add dash options for CLI.

[alanxoc3]

Related to #25. Read vim's documentation :help ---. The single dash should
mean <STDIN>. The double dash should separate options from files. If a single
dash is after a double dash, that single dash is interpreted as a file. The
single dash is normally implied if <STDIN> is not a tty. Vim has a read
script file mode too, you can pass a dash to that. Concards doesn't have
anything like that though, so it's safe to ignore.

[alanxoc3]

The STDIN thing should always go at the beginning. Partly because of an edge case concern with the current cli implementation, the "-" will not be needed/supported. Vim does it, but kakoune does not.