Added aliases to commands

Added:
- break: b
- continue: c
- step: si
- next: n
- print: p

Taken from the gdb command list

Also updated .gitignore to ignore built dlv file in cmd/dlv
Simplified help command message

* Remove hardcoded string and use command help message
* Include aliases in output
Set stderr on launched processes
Allow launching target process with arguments
Remove the -proc option and make it the default
Update README