Don't assume default branch is master per default with hardcode

[guyzmo]

when cloning a repository, use the API to find out the default branch, and use that instead of using hardcoded value master. To avoid following issue:

% git hub clone neovim/neovim
Fatal error: fatal: Couldn't find remote ref master
error: git-repo died of signal 11

making it necessary to give full options:

% git hub clone neomutt/neomutt neomutt neomutt
Cloning neomutt…
Pulling from github |████████████████████████████████| 
Successfully cloned `https://github.com/neomutt/neomutt` into `./neomutt`!

N.B.: nice to have: consider offering a way to list active branches of a repository. Maybe using a git hub info user/project command.