refactor args parsing with go-arg library

[mattlqx]

This pull refactors all command argument parsing so that flags can be mixed with positional arguments in commands.

• implement go-arg for parsing in main
• convert all commands to use go-args for arg parsing
• add descriptions to commands
• dynamically populate auto-complete with commands and usage info
• removes the need to modify completer.go when adding or updating commands

Fixes #71

This will also allow me to be able to take a string argument for #72

Zero maintenance help for both main and subcommand completion.

➜ build/vsh_darwin_amd64 -h    
vsh - Shell for Hashicorp Vault
v0.7.2-10-g3ad5080
Usage: vsh_darwin_amd64 [--cmd CMD] [--disable-auto-completion] [--log-level LEVEL]

Options:
  --cmd CMD, -c CMD      subcommand to run
  --disable-auto-completion
                         disable auto-completion on paths
  --log-level LEVEL, -v LEVEL
                         DEBUG | INFO | WARN | ERROR - debug option creates vsh_trace.log [default: INFO]
  --help, -h             display this help and exit
  --version              display version and exit

➜ build/vsh_darwin_amd64 -c 'append'                                              
source is required
appends the contents of one secret to another
Usage: append [--force] [--skip] [--rename] SOURCE TARGET

Positional arguments:
  SOURCE
  TARGET

Options:
  --force, -f            Overwrite key if exists
  --skip, -s             Skip key if exists (default)
  --rename, -r           Rename key if exists
  --help, -h             display this help and exit

[mattlqx]

I should've just looked for a library in the first place rather than spending a night trying to implement this. 😎 But I learned I guess.

[fishi0x01]

This is amazing work @mattlqx
I will restructure the documentation tomorrow accordingly.
Again, thank you so much for your contribution 🙇