Add `carton bundle --locked` option not to rebuild carton.lock

Sometimes I want to bundle tarballs of depending modules with `carton bundle` but do not want to update carton.lock. This PR adds `--locked` option to bundle so that

* do not update carton.lock,
* do not re-scan dependency of each module, use carton.lock's information instead.

In addition (not included in this PR), I am thinking of calling cpanm with `--download` option (https://github.com/motemen/carton/tree/bundle-locked-download), if cpanm --download is implemented (https://github.com/miyagawa/cpanminus/pull/118 or https://github.com/motemen/cpanminus/tree/download). If so, bundling speed could be tuned to be faster.

In japanese: http://subtech.g.hatena.ne.jp/motemen/20121005/1349439476