Implement gush b:push command

[cordoval]

The idea is to have a command that saves the long typing and also can integrate into a macro flow later on

git push -u cordoval 4-implement-gush-i-assign-username

instead just

gush b:push

auto-detects the current branch and your username repo and pushes, it has options to pass branch name and other repository name

This is useful for the first time, before we track the upstream branch with git -u x branch_name