Help flag

Created by: mcandre

Reproduction: [describe how you are able to reproduce ("trigger") this bug/issue.]

$ ion -h
ion: failed to open "-h": No such file or directory (os error 2)
$ ion --help
ion: failed to open "--help": No such file or directory (os error 2)
$ man ion
No manual entry for ion
See 'man 7 undocumented' for help when manual pages are not available.

Expected behavior: [describe the behavior you would expect the repro to yield.]

Some basic listing of supported command line flags is displayed.

Actual behavior: [describe the actual behavior, which is presented through the repro.].

ion mistakes both -h and --help for script paths.

Build information: [output of rustc -V, git rev-parse HEAD, qemu-i386 -version, uname -a, ion --version, etc.]

$ ion --version
ion: failed to open "--version": No such file or directory (os error 2)

$ which ion
/root/.cargo/bin/ion

$ rustc -V
rustc 1.20.0 (f3d6973f4 2017-08-27)

$ cargo search ion-shell
    Updating registry `https://github.com/rust-lang/crates.io-index`
ion-shell = "1.0.5"    # The Ion Shell

$ uname -a
Linux e5ff9b33d0d8 4.9.41-moby #1 SMP Wed Sep 6 00:05:16 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux