Feature/qeng 969 add modify tk config dsl method

I don't know if this a better or worse than the syntax you have here, but I wanted to see if you were aware that you could do this:
```ruby
  split_args.each do |arg|
    case arg
    when /--confdir=(.*)/ then confdir = $1
    when /--vardir=(.*)/  then vardir  = $1
    end
  end
```

I don't know if the explicitness or the brevity is better.

Also, are the command lines always inclusive of an `=`? Is there possibility that we'd get something like 
`['--confdir', '/etc/puppet']`?