Program Options: -U --username <user>

[cvvergara]

-user <user>
specifics: required ??? can it have default values like psql???
Component of issue: #43
be consistent with psql options for user

[cvvergara]

@sarthak-0415
To be consistent on how postgresql work use something like this: (default user is the login name)

#include <unistd.h>
        ("username,U",  po::value<std::string>()->default_value(getlogin()),
            "Connect to the database as the user username instead of the default.\n (You must have permission to do so, of course.)")