Use an arg parsing library for main and subcommands

[mattlqx]

There's a ton of community packages out there for handling cli arguments. There's presently the following issues in handling args in vsh:

• subcommands only recognize flags at the end of a command
• flags can't be combined in current implementation (-abc)
• main arg splitting is custom implementation
• args that accept values aren't implemented
• help strings are created by hand as opposed to coming from structure

For all these reasons, a good community library should be leveraged. https://github.com/avelino/awesome-go#command-line has a dizzying list of what's available.

[mattlqx]

I personally think https://github.com/alexflint/go-arg is the way to go here. Args are defined in a struct and supports subcommands which gels nicely with how they're implemented today.

[fishi0x01]

Good point - thank you! 👍
The amount of flags has been growing quite a bit. So an arg-parse library definitely makes sense.

I see you also already implemented this. Will have a look at your PR today.