use clap for cli argument parsing

[kinggoesgaming]

Before

hermit.com -h or hermit.com --help

hermit.com [-f <path_to_Hermitfile] [-o <output_path>]

If a path to a `Hermitfile` is not provided, it tries to load `Hermitfile` from
the current directory. If an `output_path` is not provided, the hermit is
written to `wasm.com` in the currently directly. On Unix-like operating systems
you must `chmod +x wasm.com` to make it executable. This is required because
WASI does not have a `chmod` function.

After

hermit.com -h

Usage: hermit.com [OPTIONS]

Options:
  -f <HERMITFILE_PATH>      Name of the Hermitfile [default: Hermitfile]
  -o <OUTPUT_PATH>          Output destination [default: wasm.com]
  -h, --help                Print help (see more with '--help')
  -V, --version             Print version

hermit.com --help

./build/hermit.com --help
Usage: hermit.com [OPTIONS]

Options:
  -f <HERMITFILE_PATH>
          Name of the Hermitfile

          [default: Hermitfile]

  -o <OUTPUT_PATH>
          Output destination.

          On `unix` platforms, you will need to run `chmod +x <OUTPUT_PATH>` to make it executable.
          This is required because WASI does not have a `chmod` function.

          [default: wasm.com]

  -h, --help
          Print help (see a summary with '-h')

  -V, --version
          Print version

closes: #17

[kinggoesgaming]

Waiting on #16, as it also touches the same lines as this PR.

I also need to add the chmod note to the docs.

[G4Vi]

Thanks, this looks cleaner and more maintainable than the hand parsing.

After you rebase and add the chmod note, could you please post size comparisons? I doubt this adds much, but I do want to be careful about hermit's size moving forward.

[kinggoesgaming]

add the chmod note

I added that in a14c49f, however it is only visible under --help not -h. The way we can address is when the output is processed, we can print out (only on Unix-like OSes) to manually run chmod.

Main reason for the difference between the two is that clap prints only the first line of the doc for -h and the whole thing for --help.

[kinggoesgaming]

I am going to recreate this PR tomorrow. Messed up bad