Add a git-setup command

[EtienneM]

Here is the commands added:

╰─$ cli --region osc-fr1 --app test-java-buildpack git-show
-----> Your application test-java-buildpack Git remote is: ssh://[email protected]:2200/test-java-buildpack.git

And

╰─$ git remote -v                                                                            
origin  [email protected]:Scalingo/cli.git (fetch)
origin  [email protected]:Scalingo/cli.git (push)
╰─$ cli --region osc-fr1 --app test-java-buildpack git-setup --remote toto 
-----> Successfully added the Git remote toto on test-java-buildpack
╰─$ git remote -v
origin  [email protected]:Scalingo/cli.git (fetch)
origin  [email protected]:Scalingo/cli.git (push)
toto    ssh://[email protected]:2200/test-java-buildpack.git (fetch)
toto    ssh://[email protected]:2200/test-java-buildpack.git (push)

If run from a directory with no Git initialized:

╰─$ ./cli/cli --region osc-fr1 --app test-java-buildpack git-setup --remote toto
 !     An error occured:
       fail to initialize the Git repository: repository does not exist

Fix #430

[johnsudaar]

LGTM