Tweaks to arg_hacks and add v8-flags to repl

[kevinkassimo]

Slight tweak to arg_hacks.
Now:

deno => deno repl
deno -h => deno -h
deno -L=debug => deno repl -L=debug
deno main.ts => deno run main.ts
deno -L=debug -A main.ts => deno run -L=debug -A main.ts

This is not a perfect solution since it does not handle cases like -L debug and would treat debug as the main module name if subcommand run is not provided.

Also fixes deno run without main module crash by instead printing out an error message Please provide a name to the main script to run.

Also adds --v8-flags as a flag to deno repl (can be helpful to do some command-line v8 testings)

[ry]

Can you create a function v8_flags_arg() (similar to reload_arg()) so that this bit of code can be shared between repl and run

[ry]

LGTM - thanks - I should have been more careful in the refactor