Rename CLI flags to subcommands

[bartlomieju]

This PR changes CLI flags --help and --version to subcommands.

• Changes to deno --version:
  It has been renamed to deno version subcommand, so effectively you no longer can do deno --version some_file.ts

$ deno version
deno: 0.3.9
v8: 7.4.238
typescript: 3.4.1

• Changes to deno --help:
  Deno still has deno --help and deno -h. Additionally a new subcommand deno help is present which can be used two-fold:

$ deno help // works just like deno -h
deno

USAGE:
    deno [FLAGS] [OPTIONS] [SUBCOMMAND]
...

$ deno help <subcommand> // display help for subcommand

$ deno help eval
Eval script

USAGE:
    deno eval <code>

FLAGS:
    -h, --help    Prints help information

ARGS:
    <code>

@ry if #2189 is a go, I'll add it to this PR as well

[ry]

Let’s do “deno run” in a different PR. That has a higher likelihood of breaking something.

[ry]

LGTM