Add support for command mode

[hattan]

shArg currently works with switch parameters (- or --) this proposal is to add support for sub commands.

A sub command can be along the lines of

./script.sh

An example would be

# test.sh

envCommand() {
  local commandName=$1

}

shArgs.command "env" "envCommand"

# parse inputs
shArgs.parseCommands $@

Invoked with

# invoke the env command with the reload sub command
./test.sh env reload -d