Allow launching target process with arguments

Pass extra commandline arguments to the target process so that:

`dlv -run -- -opt some_arg another_arg`
launches
`./debug -opt some_arg another_arg`

and 

`dlv -proc cmd -opt arg`
launches
`cmd -opt arg`

The second commit removes the '-proc' option and makes it the default, so that

`dlv -proc cmd -opt arg`
becomes 
`dlv cmd -opt arg`
I would remove this assignment altogether and use the result of the append in the call to `proctl.Launch`.