Add command arguments completion

[nabaco]

While typing a Gina command with arguments, allow tab completion for the arguments. Example: :Gina chec<Tab> myB<Tab> Gives :Gina checkout myBranch

:Gina push or<Tab> myB<Tab> Gives :Gina push origin myBranch

tpope's Fugitive on Vim can be used as an example.