Support the option to `get` and `import` by SSH with GitHub

Support the option to clone by SSH with GitHub.
Target commands are the following;

- `get`

It gets by SSH with GitHub if run `get` with `-p` option and the argument isn't absolute URL (means omitted).  
e.g.

    ghq get -p motemen/ghq

Then `ghq` uses URL that like so `ssh://git@github.com/motemen/ghq` instead of `https://...`.

- `import starred`

It imports by SSH with GitHub if run with `-p` option.  
e.g.

    ghq import starred -p motemen


And `-p` option has the compatibility with [hub(1)](https://github.com/github/hub).