Make Sharg completely POSIX conform

[MitraDarja]

Description

This is a followup of #95 . As stated in #95 (comment), we don't support the condition: A token consisting of a single hyphen character is interpreted as an ordinary non-option argument. By convention, it is used to specify input from or output to the standard input and output streams.

Tasks

• Add the single hyphen option.
• Delete note in documentation that we do not support this condition.
• [FEATURE] file validators should handle stdin and stdout properly #124

Definition of Done

• API documentation added