Add -alert option

[yawaramin]

Like ocaml REPL:

$ ocaml -help
Usage: ocaml <options> <files>
Options are:

  -absname  Show absolute filenames in error messages
  -alert <list>  Enable or disable alerts according to <list>:
        +<alertname>  enable alert <alertname>
        -<alertname>  disable alert <alertname>
        ++<alertname> treat <alertname> as fatal error
        --<alertname> treat <alertname> as non-fatal
        @<alertname>  enable <alertname> and treat it as fatal error
    <alertname> can be 'all' to refer to all alert names

[emillon]

Would you be interested in writing a PR for this?