Support non-incremental parsing from stdin

[bclement-ocp]

Some parsers (e.g. Alt-Ergo's native format) do not support incremental parsing, because there is no clear delimiter between phrases.
Using such formats with an `Stdin input throws an exception, when it should just switch to the full parser.

This would obviously not allow interactive input for these parsers, but would still allow piping files in this format through stdin.

See also OCamlPro/alt-ergo#949

[Gbury]

Closed by #201